Love this game
You really need to make a second one, with 2 player functions, or just a "training mode" so that you can practice without any issues.
Cheat codes would be nice, and having availability for multiple weapons would be good, like having a Freeze Ray and a Cannon at the same time, freeze + blast.
Maybe make a few more levels, like 20 or so. Perhaps some Battle Royal style Campaign battles (Boss battle with 2 or 3 enemies, rather than 1, all teaming up against you).
Hmmm... what other things could you add... How about a better level editor. Like with the Universe City level, where the platforms change every so often, give us the chance to make a level and have it change at times.
Otherwise, extremely good game. I've voted 5 on this game 4 times at this time.