I think Nolan did a giantly good job in re-inventing batman and making it so professionally incorporating so many elements of the comics, it's hard to 'outdo' Nolan's work.
What never should happen is go back the 'joel schumacher' way though.
Nolan's 3 batman movies are amazing, however, it leaves the franchise with a bit of a 'problem'.
You don't want it to keep beeing too grim and dark. Even though Batman as a character is gritty, it needs to step a bit away from becoming 'sad'.
It also needs to avoid becoming camp. It also needs to avoid being repetitive.
Batman should not reboot, and should avoid the Joker and Bane to be re-introduced. It won't work.
The Joker did end pretty well, it doesn't need 'closure' or continuation. That being said,
I can see one way of re-introducing an older, more crazy, menacing, and vengefull joker; specifically through the "
Batman: Under the Red Hood" - Joker/Black Mask scene.
https://www.youtube.com/watch?v=vay4-ieB1fk
Still, i think the joker should be left alone as a character.
Instead, the Riddler indeed would be a great personality to introduce, but please, do not use camp actors. stay away from the 'popular' actors. It just doesn't work.
The riddler could be a great character as somebody influenced or 'inspired' by the joker.
Pushing the 'detective' side of Batman to the limits.
Thing is, there's no way of knowing how a new Batman movie could be like untill the Batman VS Superman came out. How will the story be? What will it close with?
Does batman come out of his 'lair' because of superman?
If the super-film wouldn't excist, you could have the Riddler being a serious and dangerous threat for Gotham, in the style of the Joker, but worse, forcing batman [once again like with Bane] out of 'retirement'.
But would the new Batman movie be timelined before, or after the events of Batman Vs Superman?
One thing i'm more inclined to be interested in, is Joseph Gordon Levitt as 'Robin'. TDKR ending was supreme. How to incorporate that? I don't know.