Enjoyed the game! Nice controls on the tank. Simple and fun!
I will second onthezero, knowing which way the tank faces would be nice.
I think some movement differences in the NPC could be a nice improvement. I used the same strategy for all of them: moving forward in circle around the arena and shooting my pursuers. An increase of spawning shrooms once you have the latest upgrade would be a nice power trip. I would add a few elements to juice up the intensity of the combat to make it more "intense" (screen shaking and such).