You may remember that he passed on making X-men 3, in order to work on Superman Returns (an arguably much worse film). Brett Ratner stepped in and made the highly profitable third movie in the series, however many fans were upset with the direction Ratner sent the series, as he killed many main characters.
No talks have surfaced as to what kind of movie he would want to make, but it is assumed
Personally, I would love to see him tackle another X-men movie. The first two X-men films, if you take them out of the context of the comic book, were fantastic movies. Superman Returns wasn’t good, and I can say that with a huge degree of certainty, so it would probably be a good career move to return to what you are good at. I know…let’s have Hugh Jackman play Wolverine in the Avengers movie and have Singer direct it.