My guesses would be that the F4 will have SkrullJohnny. Look at how much he grew during Waid's run, and look at what a jackass he is in Millar's. Now I'll be happy to chalk this up to bad writting if I'm wrong, but it'd be a fairly easy out since Johnny is a fairly big name who's done nothing of interest since he was Galactus' herald.
On the Mighty side all eyes are on Wonder Man, because he's dating Carol (who we know isn't a skrull at the moment because of events in her own book) and sucks. After the recent NA I'd look closely at Black Widow too, but that'd make Bucky sad and so I'd rather that not happen.
New Avengers have Echo or maybe Iron Fist. He's off in some martial arts thingy and might not have made it back just yet, and she can't read lips when everyone's all masky. That I'd bet is down to some slight art errors though, but everyone else is fairly well covered.
I don't know enough about the current X folk, but let's say Angel because why not? He's out of the blue skin, and he was romancing Husk in Austen's run. Undoing a lot of Austen's bad writing might make some people very happy. On the other hand, undoing the Juggernaut thing would be more in tune with Marvel's 'everything as it once was' policy, and easy to do. It'd also suck.
Maria Hill too, or whoever put her in SHIELD.