71M, are you still working on this? So far you have done a great job but I do have a few suggestions (which I'm sure you have already heard before):
1. Lose the booster, instead just have the "ship" move with the control pad/analog nub.
2. Have the weapon fire in a certain direction depending on what button you press (Square = Left, Triangle = Above, etc).
3. More types of enemies of course.
4. Brighter bullets/enemies (more-so on the bullets).
5. Floating power-ups would be a nice addition also (shield, bomb, gun fire rate, extra cannon).
These are all my opinions so take them however you want to. And thanks for making this game!