I'm still waiting for Sega Sports' Synchronized Swimming 2k3. (Had to love Peter Moore. ;P)
Failing that, I think I could settle for Peach and Krystal's Xtreme Beach Volleyball. DoAXVB was lamented for "base lining" all of the character proportions in their title (I'm not kidding about this, either), Nintendo will fill the gap in the market by providing the "weird ones"! Easily 10 Million+ sales alone just by including Samus.
The only problem with creating a lot of Mario sports titles instead of one or two focused ones is that they could all turn out very mediocre, like the Disney Sports series that's flooded onto GC and GBA. Besides, the Mario crew just never seemed like basketball players to me, other than the odd Mario Party minigame. Still, they could maybe mix in the powerups from the platformers, you could throw fireballs or trip someone up with a raccoon tail, have more jump airtime with a P-Wing or be invincible to checks with a Metal Cap or Starman....