What If DC Had Made A Cinematic Universe Before Marvel?
The Early References - Schumacher's Batman And Superman Lives
In Batman Forever we got the earliest piece of connective tissue between two DC franchises, with Bruce Wayne saying to Dick Grayson that "by now the circus [he grew up in] must be halfway to Metropolis," canonising Superman's hometown. Then, in Batman And Robin, there was the absolute groaner from Batman: "this is why Superman works alone." Now it'd be very easy to dismiss these as corny gags on Joel Schumacher's part, mainly because that's so clearly what they were - back then just acknowledging Superman was a bit geeky, so this was the 90s equivalent of a new Marvel movie containing a nod to Squirrel Girl - but they could have retroactively evolved into early groundwork; after all, the same disconnectivity is true of early winks in the MCU (see Cap's shield in Tony Stark's workshop).
Had it been a success, the DC film to follow Batman And Robin would have been Superman Lives. This had been chugging away in the background during the Schumacher Batman years and had a fair few problems, with a long selection writers working on Tim Burton's project. The one overriding idea across all drafts (besides Jon Peters' insistence on having a massive spider in the finale) was that it would adapt The Death Of Superman, a tepidly-received comic book arc that achieved mass media coverage regardless because, well, it was the death of Superman (Warner Bros. have been obsessed with this for a long time, bringing it to the screen partially in Superman Returns and in more comic-accurate fashion in Batman V Superman).
Overall there were odd choices in what would and wouldn't be adapted - mullet and black suit were in, Doomsday was out - but what's important for here is the funeral scene; in the comics the Justice League all attend, and the idea in the film was to mirror this with a Batman cameo. Despite the doomed production being the subject of a feature length documentary (The Death Of Superman Lives, which is awesome and you should definitely check out), very little is known about this idea and how seriously it's taken. The assumption would be Clooney'd play the role, although rumours at the time suggested Michael Keaton was in talks, pointing towards a Superman Returns-esque rewrite of continuity.
Regardless of who was playing him, the intent is clear - Batman and Superman are in the same world and can crossover...