Since Storm and C. Viper's official announcement in the game, looks like the speculated "leaked" roster is true. Though heads from fans will roll because of misleading quotes from Capcom's development team. Plus, there will be more DLC characters.
However, the next 2 characters announcement is at Jump Festa (December 18). From the leak roster, according to the guy who leaked the true roster for Super Street Fighter IV had said the following will be in MVC3:
Marvel:
Taskmaster
Galatcus (Final Boss)
Phoenix (Jean Gray)
*more unsure of at the moment*
Capcom:
Mike Haggar (of course he's in)
Frank West (though now unsure)
Another Street Fighter character (It may be M. Bison or Akuma, though Akuma has some recent cameos in one of the stages)
Hsien-Ko/Lei-Lei (Fans are starting to have a heart attack on her, because they seem to misunderstood the developer's quotes on the situation for
Darkstalkers -like a certain situation we had with the Sakura interview last year

-)
However, there is uncertainity about Megaman (which version that is, though it won't be X, whom fans begged Capcom for years to add that version)
I will also say if you didn't follow the news, Capcom had stated that the following characters/series won't be in the game:
- No Rival Schools characters
- No Power Stone characters
- No Monster Hunter characters
- Tyrant and Nemesis from Resident Evil
- Demitri Maximoff (for various issues)
- Jedah
- Ghost Rider
- Dr. Strange
- Cyclops, Emma Frost, Silver Surfer, Fantasic Four, Ruby Heart, Amingo, Megaman X, Phoenix Wright, Daredevil, etc.
- Strider Hiryu
Info here:
http://en.wikipedia....sent_characters