The Old Mixer wrote:
Hypothetically, if they did get Bale, they wouldn't have to waste time with a lot of setup. They can ignore Blake potentially becoming Batman, since we never actually saw that happen (offscreen, he could still be in training, but this movie doesn't need to get into that), and just say that Bruce came out of retirement for this (whatever exactly "this" is in the context of the story).
The emergence of super-powered aliens causing mass destruction would be enought to serve as the "this".
Also, if Bale came back it would potentially open the door to other films in Nolan's Batverse, like a Catwoman film with Anne Hathaway and/or a Nightwing film with JGL.