We have managed to find a list of all the characters appearing in the upcoming Mario Hoops 3-on-3, a 3-D basketball title for the Nintendo DS by Square-Enix, the masterminds behind the Final Fantasy franchise. Surprisingly, they've chosen to include several characters from the series. Without further delay...

-Birdo (must be unlocked)
-Boo (must be unlocked)
-Black Mage (from Final Fantasy, must be unlocked)
-Bowser (must be unlocked)
-Bowser, Jr.
-Cactuar (from Final Fantasy, must be unlocked)
-Daisy
-Diddy Kong
-Dixie Kong (must be unlocked)
-Donkey Kong
-Fly Guy (must be unlocked)
-Luigi
-Mario
-Moogle (from Final Fantasy, must be unlocked)
-Ninja (from Final Fantasy, must be unlocked)
-Peach
-Waluigi
-Wario
-White Mage (from Final Fantasy, must be unlocked)
-Yoshi

Overall, it seems like a very solid lineup to me, other than the fact that Toad is missing. Whether or not more characters will show up remains to be seen, though I wouldn't get my hopes up.

Source: Nintendo's official Mario Hoops 3-on-3 Japanese website