As you all know trilogies have been apart of hollywood for quite some time now, so to figure out the plot for batmans final chapter in the nolan universe you must first understand the rules to hollywood trilogies; so here is a list and ways they could be solved:
1.Connecting the dots:the most obvious rule for the last film of a trilogy is that the finally moive must bring the last to movies together in a logical and believiable way, so knowing this we know that the new villian will be apart of batmans past present and future (thalia al gouhl!!!)
2.The ultimate challenge: if you are anything like me than you are pretty depressed about the recent comic book trilogies that have been made (spiderman, x-men etc.) but one thing they all have in common is the the final movie provides the ultimate test for the hero, for example in the wildy popular pirates of the carribean jack sparrow and his band of buccaneers had to not only fight undead pirates but the entire east india trading company(not to mention a very unhappy sea goddess)in spiderman peter had to fight the sand man, vemon, and his best friend, so it should be no suprise if we see more than one villan (an entire league of shadows and the gotham mob)
3.Therapy anyone?: another point about trilogies that not any people think about when how to probably end a trilogy (x-men im talking to you) the character devlopment. When we last left batman/bruce waynes life it was in shambles, so it is natural the this final movie will be very character driven and very emontional and not just for bruce but for every main character included
4.Some change is bad: imagine wolverine not trying to steal jean away from cyclops, or peter loving mary jane; sometihngs never change and the same goes for batman, so knowing this we can porbably assume that we will still have crime drama, we will have a new batmobile, we will have a huge wayne manor, and we will have a female lead (no catwoman tho sorry it makes no sense to add her)
5.Money money money!!!: TDK is the most successful comicbook movie ever made and the thrid highest grossing movie in the u.s., and with stats like that you can only assume that warner bros. will to there best to duplicate if not out due that success, and to do that they need the right story and villan/actor to make people run to the box office, having said that i can't see a ton of people running to see villians that are only popular in the comic book world, you have to remember that every villian in the batman movies have been on t.v. at one point or another (that includes the animated series) and due to the realism that we all enjoy i cant see nolan and warner bros. agreeing on a sci fi villian (poison ivy, hush, black mask, mr.frezze)
of course the decision ultimately comes down to what makes a great story and alot of money so my pick for the 3rd film is talhia al gohul,the riddler, and the league of shadows comment and let me know what you think