What If DC Had Made A Cinematic Universe Before Marvel?
Batman Vs Superman As Groundwork To The Justice League
In the real world, Warner were desperate to make a Batman Vs. Superman movie in the early naughties. Now that could have come out of the failure of both franchises - it would allow both characters to return to the fore in a single movie and was built around a recent hit comic, The Dark Knight Returns - but I'd wager that if they had two successful franchises the same idea would have arisen and the standalone films would have quickly built to it.
Taking that in mind, after probably another two Batman/Superman films following Robin/Lives (so 2003-ish) we'd have got Batman Vs. Superman, this alternate timeline's Avengers: the first superhero crossover film. Now whether it would have been good I have serious doubts - putting aside director, writer and actors, the special effects of time may not have been up to scratch and those other Versus movies of the era (Freddy Vs. Jason, AVP) fell into very obvious traps - but it would still be a pretty major landmark and would probably rival The Lord Of The Rings to be the highest grossing film of whichever year it was released.
After this, there's several ways things could go - trepidation around any other hero fronting a film is still a possibility - but the only real way is up. We'd see things gradually expand towards the Justice League in the late-naughties, and from there you have the DC Cinematic Universe at full strength; like with Marvel now the release slate would accelerate and JLA would rule the box office.
It's worth pointing out that this wouldn't be the Justice League we're expected next year from the DC Extended Universe though. These movies are running on from Burton and Schumacher's sensibilities, and even if new directors took over, the visual aesthetic and overall kid-friendly tone would dominate. It's probably not a drastic change from cinema of the time, but it does alter the version of heroes we get.
And it won't just be DC heroes by this point. Which brings us to the other side of the story: what would Marvel do? If the DCCU had kicked off in the 1990s, then this has a major effect on their direct competitors...