Firstly, it would be nice to have a 'levels/custom' folder to contain all the custom levels, and an in-game menu to select custom levels from (instead of having to overwriting the official levels).
Also, since playing the same basic levels over and over gets boring, there should be alternative ways to play the game:
1. Perhaps a level 'generator' in which you can change parameters (like the largeness of the asteroid field, the number of enemy teams, etc.) and play it immediately, all from within the game.
2. A campaign mode, of sorts, where you have a large map (which can be randomly generated when you make a new campaign) of asteroids to conquer.
EDIT: Err, I just now noticed the "Suggestions" thread at the top of this section.. can anyone move this post?