I think there should be a splinter faction of demons to somehow break off the Legion and Join a Faction, if the next expansion is a Burning Legion themed one
(wichever, Alliance would be better since Horde can get Ogres and Alliance guys don't really want High elves, since too much elves aren't good for anyone, will explain why i want them in the alliance later.)
Basic lore could be that a newly subjucated by the Legion race had so much willpower that even after their
subjugation they plotted against the Legion in secret, and their time came when they were unleached upon Azeroth, they broke free of their ranks and joined a faction, because being in a world all alone, with the Legion AND the horde/alliance after you, you need allies. only a fraction of them survived during that daring escape, but it was enough.
They should be viewed with great suspicion and some disgust or hate by the other races, but the faction (again, wichever it is) will deem them powerfull and usefull allies, and their savage tactics and knowledge of Legion's own tactics would make them formidable force against the Legion.
Their Culture should be one of sadistical warfare and individualism. The most powerfull live on and prosper, the weak die and are forgotten. They weren't once like that, but when they were turned into demons they were granted a hunger for battle rivaling that of a sadistical maniacal orc. They do anything to get the job done, and will not stop fighting untill the bitter end or victory, alltough all this, they retain a little reasoning, but all this vanishes in the moment they enter battle, they wont stop untill they die or are killed.
They look a bit like humanoid lizards, hunched over a bit like trolls, with red/orange/green/black-ish, demonic scales over their bodies, some spikes here and there, a large tail, and ofcourse a pair of Flaming eyes (color customizable) and a pair of horns.
During their escape they managed to bring a few battle-trained Demonic hellhounds, Demonic and savage as them.
Why do I want them in the alliance? When they join the Alliance, it won't be really a loving relationchip. More like tough love. The alliance keeps them as allies because they are usefull and deadly, and they are allied with the alliance because they need protection and they know they can't be alone. The typical Demon is a realist, and a bit of pessimist (Their jokes will be very pesimistic too, their laugh-demonic)
They could be the Weird one out, a wild card, if you will. The rest of the alliance are forming up to be very tight knit together after MoP, and the Demonic race will be the random factor. I imagine the alliance coming up with a strategy of action against an enemy, deploying it, only to see the Demons unexpectedly continuing to fight after a tactical retreat or the demons suddenly errupting from a flank they shouldn't have been in.
I also expect the rest of the alliance races to have a meeting to discuss their savagesness. Ofcourse, some leaders will like them (like Varian), some will not, and will propose exterminating them after the war is over, and some will remain silent (like Velen, who perhaps could see their use in his prophecies)
Classes would be: Warrior, Warlock, Paladin (kidding), Mage, Rogue, Hunter, Shaman (just maybe, their totems would look wicked, a remnant of their lost civilisation.) and maybe, if demon hunter comes out...scratch that, Demon hunting demon...hunters...yea...nice...not
Racials could be something mana-concerned, since they are demons and therefore like to often eat mana. Someting related to their spikes, and a tail-swipe ability, dealing some damage to anyone behind you and knocking them back a little.
Just my very strange wish. I'm not sure it will be welcomed or not, but hey, its my wish.
I'm just sick of goody two-shoes alliance races. The Worgen had potential, but they became druids and pets...DRUIDS! I just want an alliance race that kicks ass!