Here's all the achievements for MvC3:  http://www.xbox360achievements.org/game/marvel-vs-capcom-3/achievements/ 
 
A couple of things I've picked up as clues of who else could be in the game.. 'Female Flyers' requires you to use 3 flying female characters but I'm pretty sure the only ones so far are Storm and Morrigan, so I guess we can expect another flying girl. Also 'Copy This' has me a bit curious who they're referring to besides Spider-Man. And 'Fate of the Satsui no Hadou' pretty much confirms Akuma. 
 
Anyways, thoughts?

C. Viper can "fly". "Copy This" is obviously Taskmaster, who's all but confirmed at this point (especially with this MvC3 comic sample):  

I thought C. Viper could only do a boost in the air? I dunno. Never paid much attention to her. I was thinking the 3rd was Phoenix (Jean Grey), since she was on that recent leaked list and can definitely fly. 
 
Thinking that "Copy This" is Taskmaster too. Weird that they would pair him with Spider-Man. Briefly considered that it might be Venom, but I think I remember hearing that he got deconfirmed?

" I thought C. Viper could only do a boost in the air? I dunno. Never paid much attention to her. I was thinking the 3rd was Phoenix (Jean Grey), since she was on that recent leaked list and can definitely fly. "

" Thinking that "Copy This" is Taskmaster too. Weird that they would pair him with Spider-Man. "

I don't think C. Viper can fly. Can Trish or Amateratsu? 
 
The achievement 'Darkstalkers' leads me to believe there's a third Darkstalkers character. If the leaks are anything to go by it's Hsien Ko but I'd like to think its BB Hood or Anakaris.