Towers
I think that towers should have attributes, so at the end of the game you don't just have ion/rocket/slow, like make the gun turret have say armor piercing rounds, for the heavily armored creeps. or something along those lines, it just seams that at the end of the game u have everything built and there's no more strategy left, its just send and hope for the best, but if u had more options in towers at the end, it would in turn make for more strategy's and different styles of game play. like if the Gun turret took away the armor on creeps for a set amount of time, it would make them more effective at the end. Maybe put a special attribute for each tower on the final upgrade.