Why not get Tim Burton to direct and have Micheal Keaton and Jack Nicholson return for a cinematic version of The Dark Knight Returns?
Otherwise
Karl Urban: Batman/Bruce Wayne
Patrick Stewart: Alfred Pennyworth
Ray Winstone: Penguin
Hugo Weaving: Black Mask
Bryan Cranston: Jim Gordon
Charlize Theron: Oracle
Joesph Gordon Levvit: Nightwing
I would step away from the ultra-realistic Nolan vibe and instead give the film more of a Animated Sereis/ Arkham Asylum look and feel. A violent gangwar bewteen The crime lords known as the penguin and the black mask engulfs Gotham City. Batman learns that no man is an island and he doesn't have to work alone. Enter Nightwing and Oracle.
Keeping JGL would be a good way of linking it loosely to The Dark Knight and that way you get to keep a little continuity.
You could say that after bane freed everyone from Blackgate and The truth about harvey dent comes to light, crime returns to gotham and although John Blake has took to the streets as nightwing Bruce wayne returns to gotham and the quest to shut down these new ganglords begins.
That'd be my idea anyway but i'm not a screenwriter or anything.