Grifball would rock, but this is a topic for sports that should have a whole videogame for them. Technically, it already does, and that's Halo 3, since you can already play the 'official' gametype with full local, online, and ranking support.
Only thing Grifball needs at this point is it's own achievements, really. That would be pretty awesome if Bungie were to program achievements for the Grifball gametype.