Sega, Namco Bandai, and Capcom are working on a new 3DS game together. Called "Project X Zone," it is a strategy RPG featuring characters from the three companies.
Capcom is bringing X and Zero from Mega Man to the table, which may please a lot of fans of that series who have felt burned by the company lately. Their other offerings are typical for their crossover games: Street Fighter, Darkstalkers, Devil May Cry, and Resident Evil.
Namco Bandai is including characters from Super Robot Wars, Tekken, Xenosaga, Tales of Vesperia, and .hack. You know they've got to get KOS-MOS in there. No word on Pac-Man yet, but that is probably inevitable.
The Sega characters announced at this point come from Virtual Fighter, Valkyria Chronicles, Shining Force EXA, Sakura Wars, and Space Channel 5. Nothing from Phantasy Star at this point, but we can always hope they decide to dig deep instead of just tapping the post-Dreamcast years again.
Some screens and character artwork are available at andriasang, if you care to have a look at exactly who is included.