I think Fury aswell. Black Widow would be too fucking obvious, and though it would work with Cap, Fury betraying the team would make more sense, especially considering he'd be under pressure from the other government agencies as briefly shown in Volume 1
Millar promised no alien invasions in Ultimates2, and he's supposedly leaving marvel afte this run is finished. Plus a skrull traitor would be really, really shit
Remember that the traitor was talking to Giant man aswell. Would Cap really do that? Wouldn't Cap betray the team more for some skewered sense or 'whats right!' instead of being a wholly evil bastard who talks to his mortal enemy? Wouldnt that really fuck ultimate cap up as a character? Didnt Fury callousy brush Giant man off in the same issue the traitor was talking to Hank in his own home? Effective misderection? Would anyone really give a fuck if it was Wasp? What if Millar has really been body-snatched by Bendis and its all Ult. Scarlet Witch who's gone crazy? remember Batman: Hush? It may not be wise for us to be expecting something mega-fucking-fantastic because we could be gearing ourselves up for major dissapointment
"Now TV's all about format these days isn't it, and I've got a new type for you right here.
Its me and Paris Hilton driving around in a car.... Now I know what you're thinking, but she's in the boot!"
"So you see, 'Ring around the Rosey' refers to the horrible symptoms of a terrifying disease, a disease which.....a disease which....ZIM! Theres a Pigeon on you're head. You have 'Head Pigeons'. get to the Nurse before they spread to the other children."
"Get off my lawn Cookie Beast!"
--Invader Zim