Maybe you can add a special boss like Azathot, the sultan of deamons, the nuclear chaos, etc (I know he is supposed to be the most OP god in Lovecraft stories). He can be invocate when you destroy all the waves without dying, and appaear to be a bonus boss, but finally, one of the most powerful boss in Terraria, cause he is a cosmic god and kick your a.s.s easily (with nuclear style).
I think that can be extremely interessant to fight Azathot, to prove you are a great hero and god are afraid of you (or something badass like that). I don't now how he can attack, cause he is suppose to... spit nuclear explosion, or something like that... But that just an idea, so make what you want with that.