Kinda seems like alot of defense's out there, maybe you should add what type of terrain your using the map size and how many players you want availble.
Just so we can give you ideas on how to spice it up
Maybe you could add a certain map size that hasnt been used.
Maybe you could use the black walkable terrain to walk on water.
Mybe you could add something that makes it diffrent (ie. Random Units you get, Random amount of Levels you get total, Random unit spawn when bought.
Or Maybe your could have a multiple defense were once you defend one base the next one is under attack and so forth