Muy guesses are:
1)Mario Kart Online. With lots of new cicuits to play in. And up to 12 players can play online.
2) The GameQ is coming states side and will cost the same as a regular cube. This way Nintendo would discontinue the regular model asnd replace it with the Game Qs. Tough I think they would name it the Game Cube anyway. Will most probably not happened but that would be cool tough.
3) An action game developed and Published by Nintendo. That would be surprising.
4) Nintendo buys MS.

Tell me that wouldn't be megatonish.
