Well there are two reasons. The first is an interview I can't seem to find, but was posted earlier, in which both Oldman and Nolan allude to the fact that there's going to be a Riddler in the third movie. As far as who's playing it, Reese is something of a guess. His role in TDK fits the role of Edward Nigma, the guy who became Riddler in the comics.
Also, a slightly comical thing that you may or may not believe.