@Hal:
I doubt the alliance would use a WMD. The radiation released into Gnomeregan to kill the toggs was from the reactors in the tunnels and mines below the city. I think it more likely that they would drop some bombs onto Undercity, damaging it, but not destroying it.

I think it more likely, as this article suggests, that the Horde will ally with the Blackrock clan, and will then invade(my guess would be anyway) Khaz Modan, in order to connect the territories controlled by the Horde in the northern Eastern Kingdoms. That would, most likely, lead to a full unification of the Dwarves and a new war, likely across Badlands/Loch Modan/Arathi Highlands. It would reshape the power of the entire continent.

Were I king Wrynn, I would consolidate on the EK, hunker down behind my defenses(hey, Ironforge is surrounded by a natural, mountainous fortress, the campaign needed to take it would need to be HUGE), then move over to Kalimdor, retake Ashenvale, Darkshore, Felwood, Stonalton and Desolace. That would apply the same pressure on the Horde that was just applied to the Alliance, and result in the ongoing stalemate between the factions.

However, in agreement with you, I do think Sylvanas's days as leader of the Forsaken are numbered. Either Garrosh will kill her(more likely I think Bane will do it, not Garrosh, but maybe under Garrosh's orders) or she will be banished, and will take a good portion of her loyalists in the Forsaken and(my bet) take over Harthenglen, as the new Lich Queen. Although, I wouldn't count her missing sister reappearing and either talking her down, or killing her(my money is on killing her, Sylvanas's sister is supposed to be a VERY powerful mage, the equal to Khadgar's power), either way, its time for her to start coming back into the fold.

Also, kinda weird I know, but I found something...weird while going around in Feralas, a random gray book drop...written by Vholkahn, a Warlock of "the True Horde," and that he was actually a person of high station among the "False Horde." It would be my bet that the Shadow Council will play a Big role in MoP.

Another person making an appearance, I think, will be Mendiv. He has been in hiding somewhere, most people thought in a hidden room in Kharazhan, but maybe he fled to Pandaria, where he could live in peace, among beings that might be more sympathetic to his ... unusual plight. A Gaurdian of his power, near unbridled, could most certainly add a new twist to the game, one most needed.