7. Batman & Robin Hinted At A Wider DC Universe
Although we never got Superman Lives and its Batman cameo, that doesn't mean Warner Bros. failed to get any cross-franchise references into their pre-Snyder movies. At the opening of Batman & Robin there's this particularly cringe-worthy exchange, setting the tone for the horror's that follow: Robin, in awe of the Batmobile, comments "I want a car. Chicks dig the car," to which Batman unemotionally retorts "This is why Superman works alone."
Whether this was just an obvious joke by Joel Schumacher or an actually hint towards plans for a shared universe was never made clear because Batman & Robin froze at the box office (sorry) and the franchise was left dormant for Christopher Nolan. Had it been the latter, superhero cinema could have been very different; between this and the Superman Lives cameo, DC could have beaten Marvel out by almost a decade and instigated the cinematic universe concept.
Given how shlocky those later Batman movies had gotten, however, it's unlikely it would have taken off in quite the same way. Interestingly, the original script for the movie had a different comic book nod at the start, with Robin instead fawning over his new costume and calling himself Nightwing. Which is hardly better.