Personally, if they were to make a third Batman movie, I’d like the villain to be Harley Quinn. Seeing as how The Joker was supposed to return for the next Batman movie, and no actor would be stupid enough to try and fill the shoes Heath Ledger left for that role, a logical ploy would be to bring Harley in instead of the Joker. Being a different character, they wouldn’t have to worry about filling Ledger’s shoes, but on the flip side, they are free to draw heavily from the character.