Well, Mark Hamill said he was done playing Joker, so I think this was his swan-song.
He said the same thing after the first game. If they keep coming up with a good story and throwing money at him, he'll be back. I'd love for him to do a Killing Joke animated film! Heck, they might get him back for the Dark Knight Returns they are doing as they are getting back a lot of their 'veteran' voice actors for it.