A few ideas, that I do have. Consider the following:
-A pair of bosses, which resurrect the other one if not killed within X seconds.
-Creeps that mutate after being harmed/killed.
-Creeps that get more powerful the more damage they take. (eg. Faster, hp regen, etc)
-Flying units which "fall" to the ground after getting shot before taking off again
-Creep Y which is only killable after Creep X is killed.
-Random creeps: Each creep on a wave is either spell immune, fast, high hit point, high hp regen, etc.
That's all I have for now.