1. Batman's Number One
Though the Caped Crusader has an impressive Rogue's Gallery and thus an enviable pool of available options for the next film, including some who haven't yet been given a big screen treatment, the Joker's claim comes from something more important than his value to the film's marketers, or his entertainment factor: he is the perfect antithesis to Batman. Like the well-used adage that without darkness there can be no light, the best on screen relationships between antagonist and protagonist are mutually-sustaining ones: each character draws strength from the existence of the other, and in an odd way their very existence is also justified and explained. For every yin there is a yang, and as established in both the comics and in The Dark Knight, Batman and the Joker share that symbiotic relationship - the Joker exists, and perpetrates his crimes precisely in order to balance the good that the Batman brings to Gotham, and like Holmes and Moriarty, they will be entwined in that dynamic until both tumble over a metaphorical waterfall into doom. That is why the Joker's reappearance is inevitable at some point, and whichever film-maker takes on the property next might as well accept the inevitable and bring him in from day one.