The press release mentions Capcom's Strider and Firebrand from Ghosts & Goblins (this guy). Entering the Marvel camp are Ghost Rider and Hawkeye. We're sure that the others will be slowly and methodically revealed over the next few months.
Ultimate MvC3 will also have eight new stages, a spectator mode and unnamed balance tweaks. The game is set to be released this November at a "mid range price" on Xbox 360 and PlayStation 3.
Update: Announcement trailer added.
Update: No need to wait for those character announcements -- artwork for all twelve new characters showed up on Capcom's servers. Find them here.