yeah! i was hooked on Boom Boom rocket for a while
I was thinking also, if going the 2d route... of adding more planets... or even open it up to the user base to be able to edit...
so basically extract the basic sprites, edit them to suit (like skin editing), put your mod music in and then use a separate editor (with a waveform display so you can sync it to the music) to tell it when and where to launch projectiles.
I'm thinking there'd need to be somewhere to put descriptions and what skill level it is, creator credits etc so people can simply download an LHZ and drop it in the custom levels folder or something.
Great way to promote your chip tunes and have a laugh listening to it :P