Interesting. I think there is a lot of potential to this concept, like a whole campaign with supply runs, convoy escorts, heists, and more. I think graphically speaking it would look better if you were controlling individual, rotating turrets on a ship that is otherwise facing the same direction.
More variance than the kamikaze butterfly enemies would be very helpful, too.
If you want more player involvement and longevity to the game, I would definitely recommend some sort of upgrade-able shield system instead of 1 HP/sudden death for the player. Upgrading turret rotation speed, etc.
The game could use way better music which I may be willing to write!