First, it looks like an an eight player lobby for Xbox Live games has made the cut ("Who Will Answer the Call?), along with a level-based ranking system ("Combat Specialist"). A bunch of achievements make note of a "Mission" mode, as well ("Passed the Field Test," "A New Avenger," and "Welcome to Avengers Academy!").
Perhaps more interesting, though, are the hints at (rumored) unannounced characters that the achievement list drops. The "Darkstalkers" achievement, for instance, tasks players with making a "team composed of those who dwell in the darkness." Two characters from the Darkstalkers games have already been confirmed for the title -- Morrigan and Felicia. Rumors have hinted at the inclusion of Hsien-Ko, but I'm not entirely sold on that -- the third member required for this achievement could also be the announced Marvel character, Dormammu, who resides in the "Dark Dimension."
Another achievement "Fate of the Satsui no Hadou," has players deciding "who is the true master of the fist." Tipster Power Glove notes that this could be a hint that either Akuma or Ken are set to make the final cut. It could also just as easily be two Ryu's facing off. I haven't seen the final roster list myself, but I have a hunch we won't be finding Akuma on the game disc, despite standing rumors.
Check out the full achievement list at Xbox 360 Achievements. The wait is almost over -- Marvel vs. Capcom 3 is out in February!
Marvel vs. Capcom 3 achievements [Xbox 360 Achievements - Thanks, Power Glove!]