I highly, strongly, doubt Nolan would do a 4th Batman. Would I like to see it, sure, but in the event he's finished after this next installment, where does that leave us? Besides the fact that another director may not have the same grasp over the tone Nolan did, or worse even, may try to copy him, who would be in it that hadn't been used up to that point? Right now we're all racking our brains trying to find a suitable villain after what we already got in BB & TDK. You keep pushing the envelope and inevitably you run into a very strong case of "diminishing returns."
Now then, let's say we go back to a more Gothic, fantastical vision. Great, but the same questions remain, what will be fresh and new about this that will excite fanboys and the general audience alike? Cool, we can bring in Mr. Freeze again or Killer Croc, or Man-Bat or whoever, but then what? For all intents and purposes, a good majority of Batman's second-tier villains make for exciting and great comics, even cartoons. Following up one of the (presumably, if Nolan keeps pace) greatest modern trilogies with anything less than spectacular could leave Batman back in a very familiar position: waiting to be re-invented.
So then what's left? What's something exciting, fresh, engrossing, that we've never seen yet on film? How about Batman versus Superman finally, done in the correct and not arbitrary way. Or Batman in his older years fighting not only Gotham's corruption but his own inner turmoil, finally seeing what years of superhero antics can do to ones psyche. What about the climax to Batman and Joker's historic duel, which might I add, is the only conclusion that makes literal sense, a conclusion Batman has seen years in advance. Or maybe it's Batman in a period piece, calling to mind the era of a decade not so different from where we are now as a nation, on the cusp of a very unclear future. Make no mistake, TDKR is most definitely one of Batman's most enduring and important pieces of literature. And to deny it a proper screen adaptation is at the very least, blissful ignorance....