I can see reasons for every class to be allowed on every race, and I can’t see reasons they wouldn’t be. I’ll quote relevant lore sources for the classes being possible lore-wise:
Humans have been living with night elves in their park for quite a while, and intermingled even more after Teldrassil burned down. In addition, there is a flavor lore book in-game that depicts a novice mage failing to cast basic arcane spells, with their result being a druidic equivalent - the novice mage was more attuned to nature, and had druidic magic at his disposal, but didn’t know the difference. We know there’s nothing inherently preventing humans from being druids, because the Drust exist, and Gilneas had harvest witches even before they had the worgen curse.
Does this need explaining? There are plenty of shaman human npcs, mostly enemies under the twilight’s hammer, but also random bandits. They may be called different things, like “Hydromancer” “Geomancer” etc., but in the end they’re functionally shamans. In addition to that, Kul Tirans already have shamans. They’re still human even if their model differs. While Stormwind’s culture may not have shamanism as a core tenet, it doesn’t need to - we have enough examples of humans practicing shamanism, and no reason for them not to be in the alliance as well. The same was true for night elven mages - they were integrated into night elven culture despite animosity against them and literally 0 cultural place for them.
Dwarf druid: Wildhammer, need I say more?
Night elf warlock: Night elves are clearly capable of wielding demonic powers, given the amount of them that became demon hunters. With warlocks being more socially accepted since the Legion’s invasion and the warlocks’ help with the anti-legion efforts, there’s nothing to prevent night elves from wanting to learn to master that power, since it has no real prerequisites. You don’t need to be a skilled warrior, a mage, or have druidic potential to become a warlock. You just need to summon a demon or voidwalker and make a pact to get you started. You work your way out of the pact eventually by growing stronger. Nightborne warlocks already exist, and they’re not exactly culturally appropriate considering the Nightborne were grateful to the alliance and the horde specifically for helping them get Suramar free of demons. Moreover, with the night elves hanging out in Stormwind Park, they’d also be open to encountering the warlocks of stormwind, who have their hideout in a bar in the mage quarter, not far from the park (And more or less part of it given the amount of greenery).
Night elf paladin: Tyrande has this class for the majority of BFA and Shadowlands, implicitly, by the mechanics of her powers and her purpose. She’s literally a retribution paladin, empowered by the light of elune to hunt down and bring to justice those who burned down teldrassil. She’s basically in avenging wrath form for the whole time. Wardens are also debatably night elf paladins, since they were originally priestesses who started to train in martial weapons to better carry out their new jobs. The only reason I can see to not make them available is that they’d need a white elune-ish effect for their spells, but tauren don’t have different spells for being sun druids, and lightforged draenei don’t have some super-empowered light spell visuals either. Alternatively, just apply vertex color to spells, no need to design new vfx. You do it for character effects all the time.
Night elf shaman: shamans are people who commune with spirits and the elements. We already know night elves commune with spirits considering their heavy reliance on wisps, and we already know night elves can commune with the elements because there are primalist night elves in dragonflight. In addition, they can be monks, which means they can commune with at least one element, which is spirit / chi.
Gnome paladin: they have priests already and have been close allies with stormwind for a while now. It’s impossible that none of their warriors decided to try out paladin-ship, or that none of their priests decided to become more martial in nature. Not with how inventive gnomes are, and how they like to explore every option.
Gnome druid: alright, no examples of that so far, but there is a gnome who wants to become a “gnomegen” (worgen gnome), and they’ve been about as ingrained in Stormwind as night elves have been, meaning they could intermingle and learn things from each other. With there being night elves who use guns in Hyjal, how is it possible that the super-intelligent gnome race couldn’t figure out druidism in return?
Gnome shaman: goblins can be shamans, and goblins are about the least in-balance race when it comes to spiritual / elemental balance. If nothing else, considering shamanism uses the forces of nature, gnomes could replicate the effects of shamanism with their inventions. A device that creates electricity, another that melts down rock and creates lava, whatever have you. While not theoretically shamans, practically shamans, which is how you excused the existence of tauren paladins, who are theoretically druids, but practically paladins.
Draenei have the most experience with demons out of all the races on azeroth. No draenei exist who decided to study them more closely and even use their own power against them? Yes, a warlock draenei would likely no longer be able to use the naaru’s blessing, but hold up - they can be rogues, who use shadow magic, and they have Broken integrated in their society, from whom the rest of the draenei learned shamanism. Would this be spurning the blessing of the naaru? Not really, you can still worship the naaru even if you receive nothing in return. Isn’t that an even purer form of faith? Moreover, with Velen taking Illidan’s words to heart and learning that he should be a lot more practical, after seeing how Argus fared after he up and left without even fighting, even Velen might encourage the study of demons lately. If nothing else, studying how fel affects the wildlife, and how demon magic might be used to drain the fel from affected creatures instead of trying to force everything to be cleansed or killed by the Light, as Xe’ra showed to want to do. Hell, in the end, Velen even comforted Kil’jaeden, one of the biggest baddies and one of the most fel-afflicted demons out there. He even uses a void-fallen naaru’s sliver to empower the ship he lets us use (Xenedar I think?).
Draenei druid: with draenei shamans being a thing, and draenei researchers studying the animal life in ammen vale and beyond and how it was affected by the radiation from the Exodar’s remains, how can draenei even remotely be told that they’re not close to nature? Half their efforts in the starting quest chain, which canonically happened immediately after they crashed, were to save the local wildlife from being destroyed by their own presence. They even make friends with the furbolgs right away.
Worgen paladins: Gilneas’ major faith was the holy light. They have priests. No reason not to have paladins. In fact, given how long they’ve been fighting the worgen, I’d be surprised if they didn’t start their own holy order of paladins.
Worgen shamans: Worgen are essentially humans, humans are able to use elemental magic, ergo… worgen shamans. If being afflicted by fel didn’t prevent the Broken from practicing shamanism, being afflicted by a druid curse shouldn’t either.
Pandaren warlock: They can be rogues, rogues use shadow magic, they can be shadow priests, shadow priests use shadow magic, I see no reason they couldn’t be affliction warlocks, and thus, the warlock class be available to them. No cultural incentive to be warlocks, but then again there is barely any cultural incentive for any race to be warlocks, with very few exceptions.
Void elf paladin: Alright, sure, void and light don’t mix - oh, but they can be holy priests, and we even have basically normal high elves under the same race, with the option to have 0 voidyness about them. High elf paladins not a thing? If so, any particular reason for that? High elves have been working alongside humans for a very long time, and Arator is a half-high elf who became a paladin. I see no reason high elves wouldn’t be able to become paladins.
Void elf druid: Similar to the above, there are high elf options for the void elves, and high elves were rather in tune with nature even if not obsessed by it. Plus an entire faction of blood elves were botanists with very similar powers in Tempest Keep and its associated wings, and high elves used treants, with evidence of a partner type of relationship (Witherbark quest chain) rather than subjugation.
Void elf shaman: Leaving aside that there are void-empowered shamans galore in Warlords of Draenor and even in MoP to a lesser degree, void elves are capable of becoming monks, monks utilize the element of spirit, spirit-related abilities are half a shaman’s abilities.
Lightforged warlock: lightforged shadow priest is a possibility, lightforged rogues use shadow, and with Illidan basically proving Xe’ra wrong and managing to defeat the legion using his own methods, there’s no reason the Lightforged wouldn’t have at least some who explore Fel as an alternative, to mimic Illidan’s own use of “fight fire with fire”. They’re a military faction, not a religious one, and Fel is a resource they haven’t yet started to put to use. With warlocks being a fair share of the Armies of Legionfall, and Turalyon having had to work alongside both warlocks and demon hunters, and now leading the alliance, which contains warlocks from various races, it makes little sense for them to be excluded outright. Plus, if shadow+light = fel, which is how it’s been stated to work in one of the lore books as fel was given birth from the clashing of light and shadow, then any lightforged draenei using shadow magic, meaning rogues and shadow priests, would be creating fel as a byproduct. Not sure if that was retconned, but while it may not necessarily look right to have a holy-infused draenei casting fel spells, it also doesn’t look right to have a holy-infused draenei casting shadow abilities as a rogue or shadow priest, and we can do that already.
Lightforged druids: Druids are servants of life, the lightforged are the army of light, the army of light safeguards life on behalf of the naaru. Why would lightforged draenei not be capable of learning druidism?
Lightforged shaman: Same way draenei shamans exist, lightforged shamans can and should exist. I’d say especially so given that we first encounter them on Argus, and Argus is in serious need of elemental and spiritual balancing after we liberate it from the legion. The world being broken doesn’t mean much, since the Broken of Outland learned shamanism well after the world was shattered.
Dark iron druids: Come on. Firelands druids have been a thing since cataclysm. We already have a fire cat form available, and dark iron dwarves more or less have free reign of the Firelands according to the heritage quest. Why not?
Kul tiran warlock: I’ll grant you that maybe in BFA they didn’t have anywhere to learn how to summon demons from. They’re a seafaring nation with little cultural contact with the mainland. However, the legion did invade the entire world, with invasion points all over, not just in the broken isles. In addition, since BFA, they’ve had contact with warlocks and demon hunters both.
Kul tiran paladin: Maybe they don’t have a direct equivalent culturally, but they have priests, priests can put on armor and learn how to use a sword, and now they have contact with Stormwind once again. Why not have large burly paladins? Yes, they mostly worship the sea, but with how much voidy stuff was in Kul Tiras and Zandalar, and how Light is obviously very effective against voidy stuff, they’d be fools not to develop their own paladins.
Mechagnome paladin: Same reason as regular gnome paladins
Mechagnome druid: I’ll grant you it’s hard to justify, since they have bodyparts that are replaced by mechanical implants, howeeeever, those bodyparts are not technically part of their bodies, and are just gear that is attached to their bodies, meaning that they are in fact just stumpy gnome torsos, and thus entirely fleshy, if you want to argue that’s a requirement (though I’d say not given Freya’s existence, and every other nature-wielding titanic watcher). Any gadgets they have are gear, and gear is also transported to the emerald dream when the druid’s body is transported there when they shift forms.
Mechagnome shaman: similar reason to gnomes. if you want to argue for flesh requirement, there are Earthen shamans.
Nightborne paladin: They have priests, and they’re close allies with the blood elves, who have paladins. I can see the nightborne having soldiers join the Blood Knights, even.
Nightborne druids: They had druids taking care of their gardens, to the point where the royal gardner became a plant-like entity, and they’re consuming fruits of the arcan’dor, which is a tree created through druidic magic. They also have the waywalker with them to teach them druidism directly if other races wouldn’t.
Nightborne shaman: Same as night elf explanation
Highmountain warlock: Sure, the feltotem may be a warning against using fel magic, however there are plenty of examples of warnings against using fel magic for every race, and they’re completely disregarded by those who really want to practice it. So, another case of why not?
Highmountain paladin: We have tauren paladins. Maybe it made sense when they first were introduced to not have them be paladins, but by now enough lore time has passed that they should also have sunwalkers.
Mag’har warlocks: While I’ll grant that they fought against fel, warlocks, etc. in WoD, we recruit them from far after WoD’s timeline. With the orcs being killed for refusing to join the light, why wouldn’t they turn to fel magic to fight the light? Let’s say Grom forbade it - he’s no longer in charge, or even alive in all likelihood, if it even matters. The orcs of orgrimmar have warlocks, and the mag’har are more or less integrating with regular orc society, making mag’har warlocks a possibility, if not a likelihood
Mag’har paladins: they might be against the light considering their recruitment quest, however… they have priests, Tauren Sunwalkers are a common sight in Orgrimmar, and Blood Elves make no secret of their values. Why wouldn’t the Mag’har have people who are interested to learn?
Mag’har druid: Similarly, they’ve by now met the druids of Moonglade, Zandalar and Darkspear, no reason for them to deny the chance to be a druid.
Zandalari warlock: Item from all the way in Classic WoW: Zandalar Demoniac’s Wraps Demoniacs are apparently even more demon-oriented than warlocks and closer to demon hunters. While the practice seems to have been abandoned (cough blizzard forgot cough), old ways are not dead ways, and can be re-learned. Given the massive cultural shift following the replacement of Rezan with Bwonsamdi, and that basically everyone was like “Hell no” about it, why would no one in the entire zandalari citizenship, which is shown to be pretty massive, return to Demoniac practices for power?
Vulpera paladin: Same reasons as mag’har, except they don’t even have any innate bias against the light
Vulpera druid: same reasons as mag’har
Orc paladin: Same reasons as mag’har, plus even more time spent among sunwalkers and blood knights
Orc druid: same reasons as mag’har, plus even more time spent with the druids
Undead paladin: Sir Zeliek, holy priests, Calia Menethil. While the forgotten shadow is the religion of choice among the undead, it’s hardly impossible for them to be paladins.
Undead druids: The drust exist and represent the death that is part of nature’s lifecycle. I could see the undead mastering drustic druidism.
Undead shamans: Being half-dead, they have an even closer connection to the spirits, plus they’re able to be monks, so clearly shown to already have the ability to manipulate elemental spirit. Plus there exists at least one kul tiran forsaken who is a tide-sage, and tide-sages are represented as shamans in-game.
Tauren warlock: same reasons as mag’har
Troll paladin: zandalari troll paladins. distinction without a difference.
Blood elf druid: same as void elf
Blood elf shaman: same as void elf, but with added benefit of also being around shamans for a very long time
Goblin paladin: they have priests who worship the light that is reflected in gold coins or whatever. Why not paladins who champion the cause of paying your debts on time? Kidding, but the existence of priests is honestly enough to justify the existence of paladins, who are just priests who also train in martial combat and then mix the two arts.
Goblin druids: Why not? Their destruction of terrain doesn’t appear to make them unable to become shamans, so why would their destruction of nature prevent them from becoming druids? Abilities aren’t racist, maybe there are some goblins who really hate the destruction of nature.
The only special cases:
Dracthyr Death Knight - Not yet perhaps, but probably should be a thing in the next expansion.
Dracthyr any other class - I honestly don’t see why not. Yes, evokers are cool, but it is shown that not every dracthyr is an evoker, and they are not all well-versed in the same styles of combat. While they all canonically have some dragonflight abilities, you can make that like the zandalari racial, where you can choose a dragonflight ability once every week, and that is your racial active ability, instead of yet another race with “stun the enemy” as a racial. We already have tauren, kul tirans and pandarens with the same active combat racial.
Any other race demon hunter: While it made little sense in Legion, since they were all demon hunters imprisoned from Burning Crusade times, the Illidari lost a lot of forces in the Legion war, and would have no reason to only recruit blood elves and night elves for their ranks to replenish their forces. There’s nothing that says only blood elves and night elves are capable of learning the demon hunter arts. They’d have to start like the new generation death knights, with no quest chain to do but normal leveling, but that’s… fine.
Any other race evoker: Hear me out. Rename it to dragonsworn. Everything they’re capable of doing is capable of being part of their toolkit as a mortal who dedicates their life to the dragons. Hell, monks can already breathe fire, and there is at least one mage who was stated to have mastered dragon’s breath to the extent where he could actually breathe it out of his own mouth. You’d have to remove the talent that reduces tail whack and wing flap cooldowns, since those aren’t things you can do as most races, but I’m about 99% sure that those two abilities are not a core component of what it means to play evoker.
Obviously, this would be post-dragonflight, and only assuming that the dragons get their aspect powers back, otherwise I can’t imagine them being able to bless mortals to such an extent
If there are reasons why some combinations can’t happen, and they’re either from lore or from gameplay, I’d love to hear it, because I’ve put a lot of thought into what might be preventing it, and I can’t come up with anything that isn’t very contrived.