With both Alliance & Horde constantly growing and the story moving further away from them fighting each other since Shadowlands & Dragonflight, now would be a great time to introduce a new faction altogether. They should bring in the Legion (or what’s left of it) as a new playable faction that goes against Alliance & Horde. To start they could introduce:
Eredar (Corrupted Dranei)
Nathrezim
Naga
Fel Orcs
Murlocs (oh yes!)
Never hurts to dream 
No one, especially most Mag’har, would not want to work alongside the Burning Legion.
Something about countless atrocities and destroyed and/or enslaved worlds.
These aren’t Legion! Although I would play as a Murloc…
I kinda want a Void Lord or rebuilding expansion. Azeroth still suffers the cataclysm and that GIANT SWORD nobody talks about.
Eldritch stories are the best stories.
I’d rather see the Legion return as an enemy faction, rather than playable. We deprived them of Argus, and their immortality but they still control most worlds in the cosmos.
After a period of civil war and infighting, the strongest of the demons will win out and rule supreme. Maybe launch a new crusade to free Sargeras and take back everything we took from them.
Good villains are hard to find. These primalists? They ain’t it.
No thanks. I do think we need a “Legion Remnant” villain as the Legion still controls hundreds of worlds even if we did cut off the head of the beast, but we don’t need an evil faction. If anything, just add Allied Eredar and Fel Orcs with demon Hunter options.
Beat me to it. Everything here, this is it.
To be fair, the Legion isn’t all that much better…
I think Xal’atath sums it up best.
Anyone who suggests a bird person gets a nuclear wedgie.
I vote murloc.
I can agree on another faction as long as the X-faction gameplay remains.
Illidan used them in his army but he never trained them to be demon hunters as he considered elves as superior
You can’t put a fel orc/Eredar in Mardum… They weren’t part of Illidans Demon Hunter group back in burning crusade
The only way you could get other races to be demon hunters is if the Illidari decided to train more recruits again for another Legion threat
Like how Bolvar (pre Shadowlands lich king shattering helm) went grocery shopping for dead bodies after the fourth war
The Old Gods gloating over their supposed superiority is not to be taken seriously. The Old Gods were almost completely contained and Sargy laid the smack down on them. They pretend their defeat was part of their plan but they were just pulling a Zovaal before Zovaal was even a character.
And why not? While the rest of Azeroth has moved on, those still tied into the struggle with the Legion might be looking for new allies to aid in their fight.
Sargerai allied race for The Horde. They were in The Legion for like half an hour before we came down on them like a ten ton hammer. Most of them are even flagged as humanoids.
They were just scared normal people trying not to end up on the draenei murder-pile; a very real and actual object in WoD.
That’s easily done, though. Like you said, there’s already precedent in the new DK races. Just give them a truncated starter experience, make them do like five minutes of explanation for how the Illidari are training up new demon hunters to take on the various Legion warlords still making trouble, and add orc and draenei DHs.
This tbh.
Why is the forum better at coming up with WoW concepts than Blizzard?
Couldn’t a titan, or even just the doodads in a titan facility sort of, evolve mulocs? Or do we need a well of eternity for that?
I’ve seen the alternate future where Murlocs rule.
I don’t think we’re ready for Murloc evolution.
Oh yah! I can’t believe I already forgot about that quest… maybe I was repressing it. 
Oh yes! Man’ari Eredar!
I’d for sure like to see more factions added. Black Empire for example. Need lots of eyeballs and tentacles!