I’ll have fun entertaining the idea but Nolan returning to WB isn't gonna happen. At least not for a very long time.
Also JGL is now a few years older than Bale was during the production of TDKR. Wrap your heads around that one
. Right now he'd be in his prime or veteran Bats. He might even be retired.
So I'd do it like this...
Gordon/Oldman would be back as Commish. 70ish (like Simmons in the DCEU). Jim's daughter is now of age and moves back to Gotham. She has a complicated relationship with him. Jim is now remarried. Sarah Essen is his wife. Barbara doesn't like her. Her destiny here is not to become Batgirl but the Oracle.
Blake didn't become Batman until he trained first obviously. He doesn't need to travel the world like Bruce. He's the same age Bruce was in Rises and preparing to hang up the cowl. Lucius Fox (now dead, like Alfred) built Blake a few vehicles and a armoured suit that John has tinkered with over the years. Blake also worked at Wayne Manor (school for orphans) for quite sometime & had easy access to the cave from there. Recently he's been taking over for Fox at Wayne Enterprises but a lot of the kids fondly remember John. So now he's training one of the kids from the orphanage (not the kid from Rises who kept asking Blake if Batman was coming back. He's in his 20's now and a police officer). It is indeed Dick Grayson.
Organized crime is back in Gotham. The Dent Act hasn't been around for a long time since the cat's out of the bag about Two-Face. A lot of recovery time for the city after the bomb, which allowed Blake to train, get a new career, establish key relationships and of course go out as Batman whenever the city needed him. Citizens aren't really sure if he's real or not because Batman is dead in their eyes. But this keeps Batman's legacy going. Blake has a tight friendship with the aging Commissioner Gordon.
But that's just the basic setup. Blake is murdered in the first act. And nobody knows who did it or why. ENTER CHRISTIAN BALE.
Bruce is TDKReturns age. Selina and Bruce split up but are on speaking terms. She still lives in Italy. He returns to Gotham under the radar for the funeral and to figure out what's going on. He secretly reunites with Gordon with zero intention on returning to the cowl. Until...he has to by the end of the second act. Unlike Rises, he doesn't want to come back to the cowl. In this movie he meets Barbara, he meets Dick Grayson who he nicknames "Robin" in honour of Robin John Blake. The trilogy was about legacy and that won't change here. I'm basing all of this after Nolan's 2008 quote about Dick Grayson possibly being in a crib.
SO, Bruce and Gordon do put their heads together and look into a possible Joker return. But they realize quickly that it's not the case. Who killed Blake? Was it just a gangster who got the best of Batman? Or is somebody trying to draw out the OG Bats? The city needs to be protected and Robin is just starting out so he's not ready for the job. He's too young. Bale sees no other choice.
This is a murder mystery but the stakes get bigger. I have some ideas on who the villain is, but the story comes first and the villain needs to fit that story (which is how Goyer and Nolan created their old scripts). If this was a comic, I'd make it Heath's Joker (making him shoot Barbara too), but since this is a hypothetical film, i have to dig deeper.