"How do you think they will explain some of the more outlandish race/class combinations?"
Well, they could do a lot with “Class Skins”, just as Taliesin has been preaching for a long time. Even though I see limitations in that regard. Could a Void Elf Paladin who uses Void Spells and Animations still be considered a Paladin? Isn’t the Light basically the core of the Class Fantasy?
Anyway, I’ve started to come up with some ideas, I’ll begin with the Paladin:
Night Elves
Aside from Delas Moonfang, who joined the Silver Hand in Legion as a “traditional” paladin, the Nightelf priests already were very similar to Paladins in the War of the Ancients. They wore armor, used glaives as melee weapons and have access to Elunes Magic. If it’s true that Elune created the Naaru, the should at least be some connection between her magic and the Holy Light. Imagine how cool it could be to have a Nightelf Paladin with a “Moonlight” Class Skin. Basically, just look at Tyrande’s Nightwarrior.
Worgen
That’s easy. In their core, they are humans, so if they (still) believe in the Light, they could easily become Paladins in the “traditional”, human way.
Gnomes
It would be the next logical step after being able to become Priests. The gnomes had Paladins all around them since the 2nd War. And we know they’re interested in knowledge. Not to mention their newest Allies, the Lightforged Draenei, who actually seem to use Holy Light to power their machines, so there’s also a technical aspect.
Forsaken
That’s a bit tricky, yet… after Alonsus Faol as a prominent Undead who uses the Light, it wouldn’t be too weird. They could even spin it like "The Forsaken focus on Retribution. Many of them use the physical pain that comes with the summoning of the Light, to remind themselves of their former life and what they are fighting for.
That said, the most obvious answer to that would probably be Calia Menethil as a “Lightforged Undead”. If she’s able to “conserve” some of the Forsaken, she could also infuse them with the Holy Light.
Orcs
Well, this one seems outlandish to me. One solution I can think of is that some Orcs may discover that their (Fel induced) bloodlust is soothed whenever they use the Light and it helps them to reach a state of mind that is more calm and collected (which… er… SOME of them COULD prefer?)
Regarding the Mag’har, it could be the challenge to face the Lightbound Army one day and defeat them with their own magic? To show them true justice?
(To be honest, this would be the weirdest race-class-combination to me)
Trolls
Easy, just copy+paste from the Zandalari.
Goblins
Also a bit weird. Maybe the technological aspect could play a role to motivate the Goblins (“Have you seen the Machines of the Lightforged Draenei?! We had no idea you could the Light in a PROFITABLE manner!”)
Pandaren
Lorewalker Cho: “It has come to my attention that you’ve heard the story about Zao Sunseeker from Lorewalker Stonestep in the Temple of the Jade Serpent. Only a few among us Lorewalkers know that the story wasn’t over when Zao brought down the four foul suns! Decades later, his youngest daughter, Shura Sunseeker, went through the belongings of her deceased father and noticed that one of his legendary arrows, who brought down the suns, was missing. She was determined to find it, to complete her father’s legacy. And so she went looking for them. After years without results, she went to the Jade Forest and asked the wise Jade Serpent for help. "Please, I need to find my father’s missing arrow! Can you help me?"
And Jade Serpent agreed to help her. As Shura had searched all over the land, there was only one place left, where she hadn’t been able to go: The sun itself!
They flew high up in the sky, and when Yu’lon approached it, Shura saw that there was indeed something stuck in it! As she felt the heat on her fur and pulled her father’s arrow out, the Sun’s thankful fire and light flooded her body and warmed her heart. Shura felt the Light dance in the palm of her hands. On this day, she had not only found the missing arrow of the legendary Zao Sunseeker, she had gained the favor of the Sun itself! That was the Birth of the Sunseekers!”
(Yes, I’m a huge Pandaren Fan and I love their tales )