Cameo? LOL.
Re-cast the mf and have the Joker in the third film. What's so difficult about it?
C'mon man, it is difficult because it is an incredibly difficult situation, it's not like the Rachel Dawes or Jim Rhodes re-casting, or even Batman-recasting of the last movies, not in the slightest teeny weeny tiniest bit.
and I am naming that fruit and stating the obvious here, but the reasons are, because
a. It is one of the most acclaimed performances of recent times, and will doubtless be one of the all timers.
b. Sadly, the guy died, and it was his last movie.
Now, there is such a thing as talking an artistic risk, and every artist has to weigh up the pros and cons all the time, but this one is just too weighty, if you were even lucky enough to cast the best guy in the world who could replace him, there would still be a every chance that he would not measure up.
Why? Because he would have to be riffing on an already established, and very personal performance.
It would be less automatically just by not being Ledger, without a doubt, it would be a frickin' downer just watching what would amount to a pretender Joker. Because he would be, pretending to be Ledger, pretending to be the Joker. Whereas Ledger only had the job of having to become the Joker, the new guy would have the job of pretending to be two guys, forget it, no matter what, it would be a letdown.
And because the guy died, and it is a historic performance, far too much focus would be put on the new Joker, it's lesser effect would permeate the feel of the whole movie, it would be inevitable, maybe you can detatch yourself from that, but I wouldn't be able to, and I guess a lot of fans wouldn't be able to either, hence why you are the only one calling for it on this thread so far.
It would just be a bit rubbish in comparison, forget it, Nolan wants a movie that is at least as good, if not better than the last one, and that artistic risk, is no risk at all, it would bring down the movie artistically as a whole, by the very nature of the circumstances of it's conception.