I just had an interesting discussion on the WoW-Europe boards, about the possibility of WoW having a third faction.
1)It is mentioned in PC Player, that Garrosh will throw almost all of the Undead and Blood Elfs out of Orgrimmar.
2)We have known for years from the Undead intro, that the Undead are only using the Horde to further their own goals and power. now the Lich King is dead. What initiative is left for them to stay with the Horde? Particularly now when they are almost thrown out of the Capital of the Horde.
3)Warcraft 3, worked well with the three factions, and DaoC proved that 1 vs 1 vs 1 Faction(RvR) can work as well.
4)If the Forsaken broke free from the Horde, it would make sense, that the other races that would just the Forsaken Faction, would be Undead versions of the other classes. Undead Orcs, Undead Trolls, Undead Dwarfs, Undead Elfs... Kinda like with Death Knights. It would be easy to take the current races, and just make Undead versions of them. That would be the quickest way to make an entire new faction.
It's far fedged, but on the other hand, it really makes sense Lore wise. Blizzard would allow Undeads to either remain Horde and change to a different race, or go to this new faction with Undercity as it's capital!?