damn. this game has a lot of potential.
in your place i'd put some upgradeable stuff. like rate of fire, movespeed, life and damage.
also you should add somenthing like "levels" with multiple enemies.
And change the controls of the main cannon to the mouse move and click, would make it way quicker and easyer to play.
well that's all. it is indeed a great plan for a game you have here. but it need stuff added.