1) taking away the Forsaken would leave the Horde with fewer available races than the Alli, Blizz isn't about to do that. Giving the horde a new race to balance the count won't happen either, as alli would cry that they were being cheated out of a race.
2) Literally everything else in the game would have to be re-worked to accommodate 3 factions (raids, BG's, questing areas, etc) and after cata, Blizz isn't gonna do that anytime soon.
3) It would increase the workload of every expansion from this point on.
4) There are already servers where one or the other faction is non-existent, a third faction would just exacerbate that problem.
5) It makes little sense lore-wise.
I have to agree with what Urti said. Technically we already have a 3rd faction, Pandas. But you then decided when you complete the starting zone which side you want to join. I don't think it will ever happen in WoW, maybe in Titan. Wow is to old to create something that big, it would mess everything up. Warcraft is Horde vs Alliance vs Bad guys and should stay like that imo.