One of the divisive elements of Batman V Superman: Dawn of Justice was the fact that Ben Affleck's Batman had clearly already lived and fought a long time as Batman, but little of that history was explicitly revealed.
To some fans, this gave the sense of a larger universe with potential for upcoming stories, and promised that future Batman-centric films wouldn't have to take time to introduce every character and concept, and could instead step into a world where certain elements are just taken as read.
For others, it was a source of frustration that the film just assumed fans "knew stuff," and that they didn't feel they were given all the necessary information to properly appreciate what was supposedly being set up for future Batman stories.
The verdict, on balance, seems to be that it was a good thing -- mostly because Affleck's Batman was generally well-liked -- and that seems fortunate because if a rumor at Birth Movies Death is true, that "lived-in" Batman universe is already pretty big, and that same feel will continue into the rumored Ben Affleck/Geoff Johns solo Batman collaboration.
"Many, if not most, members of Batman's rogue's gallery will be making appearances in the film," writes Devin Faraci, who is becoming one of the most frequent sources of DC Extended Universe rumors. "It's going to be crammed with Bat villains, which means that just about every major bad guy you've ever wanted to see in a movie will be in this movie."
His assumption, which seems like a safe one if this rumor is indeed true, is that most of the appearances would be cameos by Arkham or Blackgate inmates, and not that an army of villains would each have significant roles in the film's main plot.
There's no official word on when a Ben Affleck-fronted Batman solo movie will come, but there's a lot of talk that given positive reception to his performance in Batman V Superman, that film's box office take, and Affleck's age, they might try and get it together fairly quickly. Affleck is expected to direct the film, which he's reported to be co-writing with DC's Johns.