One rumor that has dogged DC and Warner Bros. in the post-Batman v Superman world has been that Ben Affleck may not continue to play Batman in the DCEU. Ben's elusive replies when asked directly have poured more fuel on the fire, as well as Casey Affleck outright stating his brother won't continue. The theory makes sense — Affleck saw his first movie critically savaged (although his own role was praised). He was set to showcase his own vision as the director and writer for the solo Batman movie, until he and the studio "mutually agreed" he should no longer direct, and then saw his own screenplay get scrapped by new director Matt Reeves. It's not the best way to keep an established actor invested. If Affleck does leave after Justice League, what options do Warner Bros. and #DC have?
No More Batman
Option one is to leave Batman behind, either by him faking his own death to retire or by actually dying. This would leave us with just one official "assembled" Justice League movie with Batman but no Superman, and any remaining Justice League films with Superman but no Batman. It seems extremely unlikely that Warner Bros. would let that happen. Assembling and showcasing the entire Justice League is the whole point of these movies, and DC films with Batman still sell the most tickets (with Wonder Woman coming in a close second). No, there are still a couple ways they could include the figure of Batman in future installments, drawing inspiration from comic book stories.
If Ben Affleck leaves, and Warner Bros. decides to keep Batman, obviously they will need to find a new actor. Why not incorporate this into the actual story? In the comics, Batman (like most superheroes) has been replaced multiple times by various characters. When Bane broke the bat in Knightfall, an ally called Azrael took over the job at Bruce Wayne's request. When his crime-fighting methods turned murderous, he was replaced by Nightwing, #Batman's first Robin. Nightwing has taken the Batman mantle several times, and is the closest to original Batman. He's definitely more similar than when Commissioner Gordon became a giant robot Batman in Superheavy.
Probably the most popular Batman replacement was Terry McGinnis in the futuristic television show Batman Beyond. During the show, the teenaged McGinnis learned how to be Batman under the instruction of an aged, retired Bruce Wayne. Since Batman v Superman: Dawn of Justice already established an older, more experienced Bruce Wayne than previous movies, this could be a legitimate story development. Maybe after saving the world in Justice League, Ben Affleck's Bruce Wayne decides it's time to retire at the height of his career? After all, members of the Justice League still appeared in the show to work with the new Batman. There’s also a lot of nostalgic fan support for the old series, enough for Batman Beyond to recently be featured again as a comic book series.
Ben Affleck still has one more movie commitment after Justice League, so Warner Bros. could use the solo Batman film to set up the transition. Any of the above characters could be introduced to seamlessly take over the role in future #DCEU installments. Or, perhaps Affleck will decide to stick with the role after all, if Justice League turns out to be a fantastic, successful movie. But for now, all we can do is speculate on who will be protecting Gotham's future.