So am I the only one who felt that, despite the Joker's claims that he is totally anarchic and doesn't plan anything, that this wasn't true in the slightest? There are plenty of points in the film where it's obvious he's thought through what could follow from one action quite carefully, and has made preparations for those possibilities in advance. The middle sequence of the film makes this pretty clear. (TL;DR, just go to the last paragraph).
When Dent turns himself in as the Batman, the Joker obviously doesn't buy it. He shows no surprise when the Batbike (whatever it's called) shows up. So he attacks the police van with Dent expecting that by doing so, he can lure Batman out, so the public sees he wasn't Dent. On top of that, drawing Batman out is vital to the degree to which his later schemes fuck with his head.
Cos before the Joker even sets out to attack the van, he already has another plan ready to go: capturing Dent and Dawes. He must have planned this in advance, because he can give details on what has happened to them while in jail, and without receiving any phone call. In other words, we can assume he had this other scenario set up to test out Batman after the chase (since the Joker knows that both Dawes and Dent are both extremely important to Batman for different reasons) just in case he did manage to save Dent. (Although it's fairly clear the Joker didn't plan to kill Dent at all).
Add to that the fact that, as Gordon says, the Joker seemed to actually plan to be caught in the chase just so the Rachel/Dent scenario was more effective and he could better screw with Batman...
Either that entire middle sequence in the film hinges on a remarkable string of coincidences, that the Joker had several separate plans ready to go that just happened to coincide with the plot as it unravelled in a way that they gelled into one and delivered X even more interesting implications, or he had anticipated the relationships between the characters, the way they would react in the situation, and planned it all. This is actually one of my only problems with the movie, that for that plot to have actually been remotely possible, the Joker must have been thinking ahead of everyone about ten steps, and the police and Batman did exactly what he wanted them to at every single one.
But my point is, I don't think the Riddler would make an interesting villain in the third film because a villain with elaborate schemes has already been explored (if not to the "oooh, look how clever I am" level of the Riddler). If they did use the Riddler, unless they make incredible efforts to distinguish him from the Joker, it could come across as nothing but a repeat, and I'm not sure that Nolan would run that kind of risk.