Not a bad idea!
I'm giving this a Six cause the idea had merit and potential - but the game really isn't much. Though I love the idea of how the ammo was spent, thought the graphics were nice and even music and sound effects were decent - this game falls to one simple thing - BALANCING.
Yup, as others have stated before WAAAY to easy, one shot kills all around, no heavy spawning, just a monotonous BANG to the right and BANG to the left.
Having multiple kills was almost impossible considering you killed everything in one shot, and spawning rate was so slow. Powerups were, of course, useless.
As I've stated in many of my reviews before - if you wanna add content it's gotta have GAMEPLAY MEANING. Having a shit load of powerups is cool and all, but if you don't use them in the game than what's the point?
Now the one GREAT thing was how you reused your own ammunition - that was pretty cool. Now what I would do - is instead of giving you such a quickly increasing amount of ammo (I got to the 200 by level 8 or so), Limit the amount of balls much more drastically. like 5 or something. That way, even without a lot of spawning or extra health for aliens - you'll create harder and more innovative gameplay. Once the player HAS to start collecting the balls he's dropped, yet he can't control the amount he's shooting (as in he spits out ALL the balls at the same time) - you'll get great gameplay value.
Powerups could be adding temporarily more balls to shots - as you've done with the spread gun and such.
Basically if I'd go for a remake I'd rely a lot heavier on a minimal amount of ammo and the need to collect it!
Hope this helps, and that I've explained myself clearly.