Dean would so be Batman if possible. :P But I'm going to be mean and give him angel wings. He'll be a Supernatural type of angel but know he's Dean that shouldn't be an angel. (and yet he is!)

Buffy will be a centaur. Yep there actually is an inflatable centaur costume - apparently it's a thing.

Mia will be an English Nanny like Mary Poppins but not Mary Poppins.