The #1 biggest possibility, is Warner Bro's having someone come into the picture. And replace Nolan, to continue the series into the future. Some would say a Reboot but I don't think that's needed. Even though there's a new director, and possibly a new Batman "More then likely". Going back over the origin when you already did it with Begin's is just over kill. How many times have we seen really good movie's get rebooted or remade? In some other form or way? To many more times then id like to mention honestly.
The 4th film should lead into a "Riddler" situation. A very large question Mark campaign to draw the attention of the fan's again. Get them excited, and what better way to continue that, then to bring in someone who are loved by the fans. Your guess is as good as mine, My dream "Director/Producer" is of course Darren Aronofsky. A very fitting spot to be filled, has always been the director's chair and who would helm it after Nolan. DA is more then qualified and everyone I would think would agree with me.
The plot in the first movie, Batman Begins was the Origin. And how Batman was built into the Machine he is today. The Dark Knight was escalation, and the third movie Dark Knight Rises... will simply be "Fugitive"type theme, where Batman is basically on the run from basically the World. If you really wanna look at it that way at least that is how I see it. Year's should pass, after the 3rd movie and Bruce should probably been in his mid 30's early 40's. That would give time for Robin or Batgirl to enter the picture. Which Robin well your guess is as good as mine, if it were me id combine them all, and change the name of the character sort of like they did with Rachel as Bruce's love interest. They should press the idea that the Mad Hatter was responsible for the Deaths of many children. Including the Robin character's much younger sister.
Robin of course would step in on a one man mission to Hunt the Hatter down. The police would be some what involved but not always. Catwoman would also be tied into the 4th movie, and she would play a smaller role. Talia Al Ghul would enter the picture try to end Batman's life, she would send the League of Shadow's ninja's into Wayne Maynor. They would abduct Alfred and take him Hostage some place in Gotham. Not only would Batman have to hunt the mysterious Riddler character popping up everywhere. But he would be concerned with Alfred's safety now as well.
Batman's larget mission to date would take place, and his detective skills would most certainly come into play alot in this film. He would be challenged greatly intellectually, and with no remorse would he back down from that challenge. He would be pushed to length's never been reached before. There is also another way this could go down. They could possibly take it to the next generation of Batman, the "Tommorow Knight" Batman Beyond. Which I think this is really a pretty solid idea.
You could just start over and be done with the trilogy of the last films. It would be a good start to go by, and Terry could make his transition years later into Becoming Batman. Or you could just kill Batman off, and end the series that way, what do you think? Where will the franchise go after Nolan? Do you possibly have another idea go ahead and let me know, and as always keep the conversation some what civil thanks guys.