Exactly - if we want one that predates Marvel and ends the Nolan Batmans, we'd have to do a
lot of PODs.
The best place to start is
probably in 2002, when Batman VS Superman goes into development. Unlike our reality, JJ Abrams doesn't resubmit his Superman script; without the spoiler, Batman VS Superman comes out in 2004. This is the foundation, a film has established the superheroes live in the same world and this could lead to an organic move towards a shared DCCU. First people started tying other film - starting with
the 2004-written Jack Black comedy Green Lantern (which would be greenlight if BvsS does well) - into the first as a bit of fun, then an executive goes "we could do a Justice League film" and it becomes a bit more serious.
The problem: look at that plot for Batman VS Superman. It doesn't sound that fun and, worse for DCCU purposes, it severely messes up what you can do with a future Superman & Batman film in the same universe. (Lois and Clark frigging divorced!!) Would the film do well enough to get a DCCU? If it did, would it be a DCCU that was fully functional?