A few people have speculated that characters from Devil May Cry and Resident Evil may be making an appearance in Tatsunoko vs. Capcom because of a little blurb at the end of Capcom's recent press release for the game.
Capcom, the Capcom logo, Resident Evil, Mega Man, Darkstalkers and Devil May Cry are either registered trademarks or trademarks of Capcom Co., Ltd., in the U.S. or other countries. Street Fighter is a registered trademark of Capcom U.S.A., Inc. [...]
The reasoning here is why mention Devil May Cry and Resident Evil in the press release, unless there were characters from those games that will be making it into the title?
This is definitely speculation, as these games could simply be mentioned because they are other popular franchises that Capcom owns, and it isn't uncommon for companies to bring up games in this fashion in their press releases.
Still, since Capcom has stated that there will be over 20 characters in the U.S. release of Tatsunoko vs. Capcom — and two of their biggest franchises are Resident Evil and Devil May Cry — this rumor has some solid reasoning behind it.