I know Harvey is supposed to be the next villain or is luck not going to work out for him this time. Joker was supposed to be in the third film, there are other actors who can play the role. Heath Ledger did an excellent job and I would of loved to see him in another film as The Joker.
But what about Harley Quinn? Sure, her and The Joker do have a relationship. I think she would make a great adaption as a Batman villain on-screen. I am sure some actress out there can portray her character well.
All in all I do think she would do just as fine without The Joker, it would be fun to see what the two would do together. Quinn is a Gotham City Siren.