That’s really considering how you define species. Because if they just need to be able to produce fertile offspring together, then a human would also be the same species. And probably a bunch of others.
And humans and elves definitely are not one species.
Frankly, that’s a conundrum many D&D-esque fantasy worlds face.
For Middle-earth, Tolkien explicitly wrote that humans and elves were one species, which was why they could produce fertile offspring. It makes sense, since they’re both Children of Iluvatar, their creator god.
However, in many fantasy settings, humans and elves have completely different origins, yet still coincidentally look similar — often more similar than any other two races in the setting — for no perceivable reason. And can have fertile offspring, also for no perceivable reason.
Are they the same species? Who knows. Plausibility is already out of question.
My headcanon for WoW is that almost all titanborne uplifted, ascended or mutants across planets are fundamentally similar with intuitive understanding of Titan Common and the ability to crossbreed.
We don’t see much of some of these half-people but that’s at least somewhat due to genetic traits not being too dramatically obvious most of the time (Garona not having hooves etc). There’s a limit, though and some are too physically different to have offspring but the titan shaped humans and elves are obviously compatible.
This is my headcanon too, because I fear that with current lore, nothing else makes sense.
Sadly, I think that’s the best solution for many of the lingering question about the WoW universe’s war. The Great Beyond is held together with WD-40 and duct tape.
You can’t have tauren/pandaren and gnome/naga. Not even the titans in their hubris could predict the audacity of their children.
I agree with you, but half of Deviant Art and Tumblr don’t… 
The clutch hatches and dozens of scaled, fanged gnomelings swarm. Roll for sanity loss.
Undeath as it exists in the afterlife isn’t indicative of a life after death in death. Maldraxxus is unique in their proclivity for preserving the soulstuff of fallen warriors and giving them new bodies crafted from the flesh and bone of the organic continent they inhabit.
This new frankenstein’d body isn’t “alive” in the conventional sense of a kyrian or fae but mere Maldraxxus meat reanimated into a vehicle for a soul. As such, their bodies are definitionally undead in a realm where everyone already died at least once.
Some outsiders pursue this peculiar immortality for their souls but in the process inevitably join the ranks of the realm’s unliving legions.
At some point I’ll make sense of san’layn vs. darkfallen. At some point, I’m sure.
New lore dictates that all undead and risen elves (at least ones looking like elves instead of ghouls, zombies, ghosts etc.) are darkfallen. But so too are the san’layn, who appear to look like either Death Knights or vampyr from Wrath to BfA, and whose main difference seems to be literal blood magic in the vein of DKs.
Or am I totally lost here?
Headcanon-y solution: darkfallen are just another example of elves mutating into some new, interesting subgroup by exposure to magic. In this case, necromancy since they’re a uniquely elven undead variant.
All undead elves are Darkfallen, not all Darkfallen are San’layn. Rather the San’layn are, by all accounts, presumably an order of undead elves afflicted with a strain of vampirism.
Since the Third War and rise of the Scourge, most have changed their burial ways to deny the vile enemy the use of the bodies of loved ones, be it through cremation or other means.
Coincidentally, a small industry has popped up where battlefield salvagers collect the gear of fallen soldiers for a fee. Others keep it, being considered looters or adventurers but either way, bodies go missing too.
Some speculate these dead end up sold to the forsaken or ebon blade, others suspect that these looters have ties to pet food manifacturers.
They are vampyr. Basically vampires. They are also darkfallen by virtue of being elves in life, but they’re a specific type of undead with inconsistent visuals. Not all vampyr are darkfallen and not all darkfallen are vampyr.
Mermaids are or I should say were real.
however they are not native to Azeroth, not entirely anyway.
when the titans came to Azeroth as they had many other worlds, one of them moulded a servant species based off a race of mer folk they encountered on a pervious planet.
these mer-maids and men would serve a forgotten titan keeper tasked with work under the water, partly some of it was keeping Neptulon and the elementals in check.
As is known the old gods unleashed the curse of flesh, corrupting the titan forged races and the mermaids were no exception. this no doubt caused them as much distress as their land bound counterparts, what is worse, not only did they lose their immortality over time, when the sundering broke the land a part, it killed a great many of the mermaids, leaving only a handful scattered groups, even worse yet, the Naga now raged war lowering their numbers even further.
One of the few bastions of Mermaid society happen to be just outside of the islands which would later become kul’tiras, living so close to the coast they were obviously seen by the early human settlers, who knew them to be as real as the seagulls and murlocks.
However as time went on and Kul’tiras grew into a mighty nation, the sightings become fewer, unbeknownst to the surface dwellers the last bastion of mermaids had fallen to the Naga and they were seen never more.
Contrary to the popular belief of a particular lupine dark sorcerer, mermaids are still very much around albeit in waters yet to be explored.
Originally created by Golganneth alongside the sea giants, the original mermaids resembled nothing more than vrykul with fish tails and while the sea giants dredged out the seas, the mermaids acted as both pest control and messengers as to ensure the sea giants’ work was carried out without distractions.
A sub-caste and the only remaining tribe of mermaids were tasked with keeping a direct link of communication between the Dragon Isles and the rest of Azeroth utilising a type of bioluminescent water magic, but following the curse of flesh and the workings of N’Zoth, they went into stasis.
Fast forward to the near future and the mermaids have awoken from their slumber following the defeat of N’Zoth and once more harness their bioluminescent magic to signal the watchers on the Dragon Isles that Azeroth is ready for the second coming of the dragons - Koranos being the sole watcher to respond to the luminous message in the ocean waves.
Orcs, with their dense bones, natural tendency toward muscle bulk and Breaker standard hyper efficient digestion must eat a lot to maintain their physique.
As designed by Aggramar, Breakers were made to grow, destroy and consume leaving only desert. The interference of spirit energy gave these constructs souls, reason and some degree of moderation meaning they created something for themselves beyond their purpose but still existed in a cycle of growth, destruction and consumption. Left to their own devices without reason, the Breakers would’ve continued destroying draenor until nothing remained to consume but themselves.
Instead, the energy efficient orcish descendants maintained a society after a fashion but some like the Bleeding Hollow would still cannibalize other sapients, the Bonechewers in particular finding that dense, orcish flesh was highly nutritious and perpetuating the Breaker cycle to its eventual logical conclusion of eating their own to death.
To elaborate on a point made elsewhere:
As sailing has been the go-to ship design since antiquity and mechanical solutions were only invented around the second war (submarines, paddle steamers), the growing navies of the world quickly caught onto the ancient ways of using magic to guide ships across the seas. A rich tradition exists of employing mages to manipulate the sails with a freezing gust where the wind won’t suffice. Thus keeping a steady pace.
Pioneered by the ancient kaldorei, the task was borne by heirs of the sorcerous highborne too removed from power to hope to inherit, taking to the seas for adventuring. A few families took this to heart and proudly made themselves known as seafarers and explorers.
In relatively modern times, it was the quel’dorei who brought the tidemage’s craft to the eastern kingdoms and it’s been adopted across the human kingdoms as knowledge of the arcane grew. The Gilneans with their peninsular kingdom took this most seriously, producing highly trained adepts to guide their vessels in colonial enterprises, truly ruling the waves for a time.
The duty was difficult and serious, taking on aspects of disciplined ritual over generations as honoured naval tradition and eventually a new religious movement rooted in the Old Ways of mankind’s reverence of nature. By the time the colonization of kul tiras became a reality, these tidemages had become tidesages with an entrenched devotion to the Sea as Mother, alleging to hear Her voice in the waves. Some were in fact correct for one reason or another, manifesting as non-arcane command of the waves and modern tidesages are internally an eclectic mix of shamans of the sea and religious wizards.
The other kingdoms did not treat such peculiar devotion as acceptable given the Light’s cultural dominance and so their tidemages, squallshapers or whatever they may be called locally remain a useful but secular addition to the crew.
The Zandalari for their part commune with the spirits of the sea, disdainful of the younger creatures of Azeroth for their lack of faith.
Headcanon: Warcraft has an equivalent of the Hold Person spell, and its verbal component is “ENOUGH!”