Not going to happen.
There are way to many game play issues involved and way to much work to be spent for virtually nothing. The only thing more than 2 factions will accomplish is less people to group with and a different tag.
It also hardly makes any sense lore wise. Forsaken going out of the horde would make some sense but any race allying with them won't, because if they want to raise everyone as undead, the rest are living you know. It would be both easier and much more convenient for the living races to destroy the undead and their "corruption".
What's up with the 3 factions nonsense anyway?
Oh, and about World of Starcraft, that won't happen anywhere in the next 3-4 years if ever. SC2 already has another expansion planned StarCraft II: Legacy of the Void.