Just a couple of things I think this game needs:
1) Upgrades (Earned via cash or perhaps there's an in-game upgrade system like that in Raiden X)
2) More sounds! An explosion when you lose a life and a pewpew when shooting.
3) The enemies need to shoot back, I found it was too easy to stay alive.
4) Improve the graphics a bit
5) What happened to S.G.A.S.S 1-3? Why did you jump straight to 4? Unless I'm missing something...
6) Levels! I find high scores to be a terrible incentive in a game. When I'm competing with only myself, I really don't care about my score.
That's all I can think of right now.