After Justice League, we aren't sure where we're going to see Henry Cavill's Superman pop up next. Rumors have been coming out that he would appear in next year's Shazam!, and Cavill himself has commented that he is seeing a Superman/Captain Marvel/Black Adam crossover down the line.
When Cavill was asked by MTV News if we would see Superman and Black Adam eventually clash, Cavill said:
"There is definitely some kind of work towards that. There's a lot of things spinning around in the universe right now. Obviously, if Black Adam does become a character officially [Dwayne Johnson] will play him, but I'm pretty sure it'll be a Shazam/Black Adam thing first. If it does happen it would make sense that way, but you never know. Maybe it'll be a Superman movie, and Shazam will come and help—that's kind of stealing his limelight though which is… not cool."
For now, Cavill has been open about renewing his contract with the DCEU to play Superman, and rumor has it that he wants this appearance in Shazam! to be his final Superman portrayal before jumping in on a new contract. With Ben Affleck apparently unsure whether he wants to keep playing Batman, at least Cavill is keeping positive about being the Man of Steel—for a few more years at least.
For now, people are confident that Cavill will appear in Shazam!. Though more people are banking on an actual meet-up with Captain Marvel, I think Superman's involvement would be more of a perspective from Billy Batson's eyes (e.g., Superman on the news or in magazine clippings).
Shazam! comes out April 5, 2019.