Personally, I think that he should have stayed with the X-Men franchise. It would have spared us that god-awful X3 crap.
Speaking of DC Comics pics, it is highly unlikely that Bryan Singer will return to shoot the next Superman movie. (The director is finishing up Tom Cruise's Nazi film Valkyrie, and prepping The Mayor of Castro Street). The next Superman we will see on the big screen will not be Brandon Routh, but a younger Superman among a cast of youthful superheroes in The Justice League. That movie will likely not be shot, however, until after the WGA strike is resolved. Warners is so happy with Dark Knight that their fondest hope is that Nolan will return to do another Batman.