It's time for a Third Faction. The Demons.
The Burning Legion
There are enough Demon Classifications to have all you need for another Faction. The other side, where the Demons come from can be the starting zones, new worlds(dimensionS) You progress as a demon/class of your type. Infernals can progress up to abyssals, maybe their advancement is through Demon types instead of classes themselves, to give a whole "otherness" feel to the progression, which the mechanics are already built in to the game. Your character skin could change as you level through the types, or maybe the choice to progress as a Class type.
Classes available:
Warriors
Rogues
Mages
Warlocks
Hunters
DeathKnights (Much-maligned and removed "anti-paladins?"
Priests
Races:
Satyr
Infernals
Imps
Succubi
Felstalkers
Felguards
I guess I just want the maintenance to be done. This has probably already been brought up.
This leads into "Too long, did not listen." comments below here from further pages, some read first and last.
It would be another expansion possible, If they can build new zones and revamp the world for an expansion, create outlands new races and classes for previous expansions, Why not? I forget what the dimension Demons come from is called, but why not an outland style set of zones for low level progression of your toon exists, Instances and zones that are enemy to Horde and alliance in outlands and azeroth are friendly to the Burning Legion. Yes, go through outlands killing Alliance and Horde, Most of the battlefields that are in contention there do not change or have story outcome, why can't the Burning Legion story outcome progress as well. I don't expect and "end" to the game, so why expect and endgame with the defeat of the Burning Legion. The Horde are Neutral as others have posted here, Now you get to play the bad guys.
I can imagine quests sending you to different locations in Azeroth/Outlands as a starter to Burning Legion footholds in those lands, carrying out questlines that Horde and Alliance are fighting against currently. I think it would be a blast to play out the infernal attacks on Horde and Alliance in Hellfire peninsula. With the new advancement in zones that Blizzard is using, your goal could be to flatten 3 buildings as an Infernal.
The framework exists, the armor skins exist, but a new set with demonic caste would be a nice alternative. The skins for all the demonic types and progressions exist as creature types from around azeroth and outlands. For class progressions, make em match what exists, or as new classes, talent trees can be either a demonic progression, or a class talent progression. Take points in being an Imp, or progress to being a Doomguard(example)
Page 4 comment
I like the idea of being fledgling Satyr huntard with my Fel Stalker pet getting the quest to corrupt the Gnarlpine Furbolgs. Quest starter in the Demon homeland, You must go to Teldrassil and meet Zenn Foulhoof. Zenn is the starter for a questline to kill and corrupt the Gnarlpine Furbolg, as well as the starter for Zenn's bidding, and Webwood venom, etc. Using the new phased approach, this faction need never meet Alliance faction players during this starter phase. Questline completion leaves the Gnarlpine in the state that the Night Elves find them for their starter quests. There are examples of this possibility all over Azeroth and into Outlands and Northrend.
Possible, just as any other "new races" and storyline for alternative expansion yes. I agree with all of you who have said, probability is nil.