I'm kinda torn on this -- I think Darion and Tirion are the most likely candidates. Of the faction-based characters, Varian or Thrall would be the next most likely. I don't really wish it on either of them, though -- Thrall, because he's too awesome and deserves better, and Varian, because frankly I don't want to see him have that central a role in all this. Nice to read he might actually show some compassion and step back from "ALL 0RCZ MUST DI3" mentality for a second.
Sylvanas is probably the best equipped to do the job, but ostensibly we already know she doesn't, because doesn't some of the Cataclysm literature have her leading the attack on Gilneas?