Using some of the other comments I've seen with some of my own ideas.....
With xxxxxxx RP you can "Re**something**" in which the game "soft resets" and you have to re-unlock all the structures. Each tower gets a new buff or something that is upgraded with RP. Including some form of a 100% chance to instant spawn mix in a random tile placement restrictions so that you can't simply rinse and repeat the exact same layout every time. Not sure what's possible but another addition would be to have turrets aim at target that you click to add some "control".