I don't think there's any way that could happen without all of the ancillary conditions being present. For one thing, you would need a bunch of distraught mob bosses willing to follow a guy in make-up and that, by itself, is enough to insure such a scenario would never become reality. Furthermore, you'd need a police force as morally vulnerable as Gotham's as well as a citizenry so concerned about their city, for better or worse, that they would be willing to, with any concerted significance, turn against the one thing that stood in the way of a Jokeresque figure taking control (be it Batman or Harvey Dent), and I just can't see the citizens of a city being that concerned, yet so easily swayed by a figure like the Joker. Apathy and the innate human drive to be self-centered preclude this. Nolan et al have done a terrific job of making a realistic version of the Batman story, but it's not so realistic that it could happen in real life, in my opinion.