Hi, I missed the start of the jam too, but I've already got a little prototype working (move, shoot, destroy buildings). It's going to be 2D top down (like GTA1 and 2) and pixel-y. Just a tiny project, nothing big.
A music composer would be nice, but I'm not sure the game will be big enough to need more than 1 or 2 tracks (for menu and in-game). If you are interessted I can send you a link to what I have. Or screenshots/video.
And maybe you can help with some ideas for the game too if you like.