It's really not so much that the Legion
sent Balnazzar and Mal'Ganis to Azeroth, it's more like they're agents of the Legion, and they're here just doing what demons who work for the Legion do(Cause turmoil).
I don't think it's a stretch to say that the Legion has been crippled for the most part following the events at the Sunwell. While I can't get into Icecrown to confirm it just yet, the Legion's presence in Northrend is....well..non-existent. Mal'Ganis is the only agent of the Legion we know about, and even he doesn't have an army of demons supporting him, his support is in the form of Humans he's manipulating(Scarlet Onslaught).
Regarding Sylvanas and the New Plague they've been working on. The Plague itself is supposed to destroy the Scourge, as we all know. It's the second part of the plague I'm a little shaky on, we all know that the Forsaken has been using all sorts of different brews of poison and testing on captured Alliance members(Tirisfal/Silverpine/Hillsbrad), and the idea is definitely to wipe them out along with the Scourge. I've never actually seen the Forsaken use their New Plague on their allies, but we know that they're not to be trusted either within the ranks of the Horde(
Infiltrator Marksen - WoWWiki - Your guide to the World of Warcraft for more fun with this one). So we know for a fact that the finished product is definitely deadly for Scourge/Humans/Dwarves, we don't know if the Forsaken would be crazy enough to also wipe out their allies at the scene we see described up above. It would be beyond awesome if it did, and I'm sure it would happen if this was Warcraft 4 :P, but for this MMO I don't see how it would work, your faction leader making a move of this magnitude is something you'd
have to support, especially one so highly revered as Sylvanas.
Speculation part:
I imagine the event in question occurs at Dragonblight's entrance to Icecrown since a huge event is supposed to take place there, featuring multiple major lore characters.
It would be beyond amazing if she DID wipe out the Scourge/Alliance/Horde forces in this scene, of course, based on what I said above, I doubt that will actually happen. There's so many ways you could go with this if it happened though, my head spins around the thought of it.
It would be amazing if she wiped out the Alliance/Scourge forces, and not the Horde, this event would act as a catalyst for a new war between the Alliance/Horde forces, could you imagine the absolute pandemonium this would cause? The Horde would be seen as much as a threat to the Alliance as the Scourge, not to mention the inner turmoil it would cause within the Horde with the Forsaken unleashing a weapon of such magnitude.
If she wiped out just the Scourge forces, it wouldn't be a total loss, because we know that she could wipe out the Alliance forces there too if she so wanted. We've seen small portions of Sylvanas's morality come to the surface at times, but it's been hard to imagine a happy ending for her in any way. Her path is one of the hardest to read in WoW, because she's cursed to be Undead, we just don't know what she's capable of doing or how her hatred for the Scourge will twist her judgment.
Anywho, I'm done ranting I think, that is until more lore-related stuff leaks.
