A somewhat darker Superman would be welcomed by me. It doesn't have to be goth- "OMG my life sucks balls"-dark, but making Superman a bad-ass mofo would be a nice change of pace. The only part in Returns that I enjoyed was where he gets shot in the eye. I was all like "Yeah bitch, fuck him up!".
I can't remember a movie where Supes isn't basically prancing around being invincible. I can't think of anyone who would act so gay if he/she were invincible (save Kryptonite of course). So yes, a grittier, darker, more believable Superman PLEASE!
As for Bats 3, as I understood it, Nolan had no intention to make a third, except if he got a good script/scenario. As for possible villains, there aren't a whole lot to choose from, since Nolan has Batman so grounded in reality that Penguin or Ivy will be almost immediatly out of the question. Then again, Scarecrow was a pretty supernatural character in the comics too, AFAIK, and he managed to give it a nice twist to ground him.
If Robin is going to be in this, I think he's going to be more of an ass-kicker than the whiney SOB he's been in previous films. Hell, maybe Nolan skips directly to Nightwing.