What If DC Had Made A Cinematic Universe Before Marvel?

The Immediate Future - Sequels And Easter Eggs

Whether you buy the Keaton re-recast or not doesn't really matter, because the next Batman film after Robin, Batman Unchained, was going to tie together all the films through a Scarecrow-gas induced vision in Arkham Asylum where Bats would see visions of his formers foes (most famously Jack Nicholson's Joker). The continuity between any and all of the pre-Nolan Batman films was loose anyway, so this crossing fits nicely and would ensure some future for the franchise.

If the idea of having a proper shared universe had taken hold at DC then we could have had more Superman allusions in there too, but it doesn't really matter - as we now have a Batman and Superman series running concurrently, assuming no halt, it will happen eventually.

As for when, that's a different story. In the late-1990s this is unproven territory for comic book movies and, unlike with Marvel (which I'll get to in a minute), there's not quite the same obvious advantage to do so; a crossover is making one movie instead of two. However, once the ball's rolling it will happen; they're not the best examples, but look at other versus films like Freddy Vs. Jason and AVP, which all evolved out of relatively simple beginnings around the same time.

I'd still expect a few more sequels for both Bats and Supes before anything major happens, but over that period you'd see those earlier mentions and cameos gradually (and slightly randomly) increase. The development would be slow thanks to the once-held rule only Batman or Superman can front their own movie, but the prominence would slowly increase; nods to Star, Coast and Central City, maybe minor cameos from some heroes (probably The Flash first, then expanding to Wonder Woman, then Aquaman and Green Lantern).

And where's it going? Why, where we've ended in a round about way: Batman Vs. Superman...