I'd love to see MM in the DCU filmverse - for me, more as Mr Freeze (I've made some manips of Woody Harrelson in this role, and I think either of them would be great) than Joker, but I'm sure he'd be a great Joker.
I hate to say it but I really have come around to thinking of Leo as the best possible candidate for Joker. And I still really like the idea of Simon Pegg, but with Joker really as a super colorful character who doesn't get much screen time. I think it would be refreshing to see a Joker who is in Arkham and doesn't have some "big plan" - just a couple very memorable scenes, perhaps even helping Batman out with something and denigrating the other villains, as has been effectively done in the comics at times. About as big a role as MM had in Wolf of Wall Street. Extremely pivotal/memorable but not at all the main action.
I don't ever want another big Batman vs. Joker style film, and I'd be plenty happy if we never saw Joker on screen again...
But if we had to, MM would be just fine and dandy. He'd probably be great in the cameo role described above, too.