Hey,
I had a couple of thoughts i would like to share and ask opinions of.

Super Tux Game Idea:
We all know or should all know Super Tux... my game idea is to make it 2-4 players. Like the new Super Mario Brothers Wii. Adding a co-op like feature could make it fun and add a little variety.
For example: You could add some of the other opensource mascots and co-op partners to Tux. E.g. Beastie, Gimp, Mozilla, etc and maybe give some team-moves, that vary according to the partner its performed with.

The other Game Idea: Fighting Game
Was a fighting style game, take for example Smash Bros, one of the best know multiplayer fighting games.

There are so many opensource mascots out there, that creating a character roster wouldn't be hard at all.

I think that a lot of people/ developers included would have fun designing moves, levels, etc for characters. Not only that but it would be such an easy game to expand, and allow the "opensource community" to add-on features to in a similar manner to SuperTux Kart.

Please provide other ideas too, this doesn't haven't to be just focused on my suggestions