Boomerang Tank
It'd be a level 45 tank, but I'm not sure what it would upgrade from. It would have sort of a crescent-shaped barrel, and would shoot spinning boomerangs. They would travel for a bit, and then come back to the player. Unsure of what the stats would be, but for those of you who play other games it'd work very similarly to the My Reflection item from The Binding of Isaac.