Re: Has this been done too much?
Just write. Ignore what demographic it might be for, what the cover might look like, whether or not such-and-such is too much like such-and-such from that other thing, and just write your story. I think too much emphasis is placed on all the behind the scenes stuff, and people are forgetting that what we do is spin stories. If we write the best stories we can, we can sleep at night.
Incidentally, JK Rowling didn't write to demographic. She just wrote the best book she could, and the publishers decided who to aim it at.