What If DC Had Made A Cinematic Universe Before Marvel?
The Competition - Marvel Struggles
Now let's look at where Marvel were in 1997, when this alternate timeline kicks off. Cinematically, they were a mess; the only major release was Howard The Duck and, bar the once popular The Incredible Hulk TV series, their screen output held no promise for the future. Financially things were worse; thanks to the burst of the spectator bubble (in the 90s comics doubled down on the adult collector market with merch and a host of #1s to drive sales in the thought people could get a modern Action Comics #1, only for the whole thing to explode in their face when people realised collectibles are worthless when mass produced), Marvel went bankrupt in 1996 and vastly rethought their approach.
They sold of the rights to Spider-Man, Ghost Rider, X-Men, Fantastic Four, Daredevil, Hulk and Blade, which in our reality meant that when the company got back on its feet in the mid-2000s it had a B-list set of heroes to play with, something they steered into, leading to what we now know as the MCU. However, in our imagined world where DC reigns, those restrictions would mean Marvel couldn't mount a counter offensive.
We'd have got movies based on X-Men, Spidey et al, but they'd be in the shadow of a much more accomplished series and the head company could do little about it. X-Men could meet Fantastic Four, Spidey could swing by Ghost Rider, but there'd be no cohesive world due to those rights issues. The company could kickstart a push towards becoming their own studio, but if a rival cinematic universe was already going it'd be hardly to build interest in lesser heroes like Iron Man and Captain America, who then had next to no mainstream popularity. Of course, we know it is possible to turn things about, but by the point they did get Iron Man out, DC would either be massive or have tanked the whole idea.
In essence, Marvel would be this world's DC, except instead of it being due to extensive false starts (and I'm assuming what Marvel would make playing catch up would be good - it could be awful) it's down to pure and simple inability. If DC got their first, Marvel could hardly respond, something I think played into their real-world decision to push for The Avengers so quickly; they needed to get their first to make an impact. After all, in comics DC are the ones all about continuity...
Here's the strangest thing about Marvel's dominance and proliferation of the shared universe concept: it really shouldn't be them. In print, DC have always been the publisher where continuity, crossovers and corrections aren't just common, they're the point; ever since the Golden and Silver Age Flash met up in 1961, the wider storyline has been all about consolidating its various runs, with the Crisis normalising events a staple. It's so integral that while The New 52 was intended to be a move away from that, it's been undone after only five years with Rebirth.
Yes, Marvel Comics were incredibly forward in how their characters lived in the same world - The Amazing Spider-Man #1 had Peter Parker try and join the Fantastic Four - but I'd say DC are the most obvious ones to bring that to the big screen.