You know, I like this idea. Even though I don't see it happening, or Nolan allowing his universe to be used in such a way...I think that's great man.
If WB really wanted to, they could easily bring Bale back and eventually re-cast him. Using TDK-T as the beginning of a very long franchise that continues like the Bond films.
Like you said, use Blake as a temporary Batman successor...you don't even have to show him in the cowl. Kill him off, use his birthname to give the "Robin" moniker to a young Dick Grayson. Luring Bruce back to Gotham after a couple of peaceful years outside of the city. He's back refreshed after a couple of huge Justice League movies. By the time he's back you have a re-cast for Bruce, a new director, new Alfred (who could get killed off in an upcoming solo film), a new Gordon. Lucius Fox could retire during the JLA saga so you don't need him in the future films.
A new more fantastical look, suit & rogues gallery for this veteran Batman who is in his 40's.
Grayson is Robin. We get the return of the Joker played by someone else..as you said.
I know some people don't like that idea..but it would be a smart thing for WB to do. Nolan would hate it, but I say who cares. It would never have to be rebooted, all in loose continuity. They could eventually do Batman Beyond with Terry as well.
Thanks. However, I initially wasn't thinking that Bruce would be 'refreshed'. I don't really see how rest could repair his cartilage, etc.
I imagined that he would form and participate in the JL out of necessity (Blake being busy or out of commission). If he doesn't, he believes that he won't exactly have a normal life to run back to; there's no real option there; he's not only suiting up to save others but also to save the future Alfred wants for him. Would Alfred be disappointed in him for that?
(this sort of conflict never existed before for Bruce, as he never had to fight crime in Gotham to ensure his own safety. However, that's a different story with a worldwide, humanity-ending, JL level threat.)
That being said, in the JL movie(s), with his regained 'will to live'/'fear of death', perhaps he makes sure to be hyper-prepared, using his head to avoid using his body as much as possible (pretty much the opposite of the Bane scenario), always with the right gadget, as not to get himself into situations where he gets beat to a pulp. He can also have an improved suit, just in case, doing as much as he can to honour Alfred's wishes, while still doing what's necessary.
However, given that a brittle Bruce is probably limiting for the future, in the next solo Batman movie perhaps get the supernatural in there right off the bat. After an opening action sequence ending with Batman's death (Blake), have Bruce (the new actor) waking up in the mountains (ala Batman Begins) face first in the snow, unsure of how he got there; his hands are bloodstained. In a medical exam it's revealed that his body mysteriously has none of the problems listed in TDKR. On his journey home he is informed of Blake's death and instead returns to Gotham. In the many nightmares he has during the movie he remembers that he was healed in a Lazarus Pit, captured by LoS and Ra's, who is still alive (a mirror to BBegins). And given the recent 'death in the family', have the movie be about death/life, letting go, etc; whether the lazarus pit should be used or not. Oh, I think the title "Batman Reborn" or "The Dark Knight Reborn" would be especially fitting here.
And 'Storyteller', yes, a reboot would be simpler. I'm just saying that if they did stick with TDKT, this is how I'd do it.