Here's my idea:
Remember
THE SAVAGE TIME? Of course you do. The JL goes back and re-fights WWII to stop Vandal Savage from taking over the world, and they team up with the Blackhawks and Sgt. Rock along the way. It's amazeballs.
Okay, now suppose they want to do that story on the big screen (it's an easy sell; did I mention the JL re-fights WWII?) Only instead of the Blackhawks, they team up with...
...the
JSA.
That's right, as a direct result of Savage mucking about with history, the JSA gets formed specifically to fight him. Of course it still takes the League to finally tip the scales in the allies' favor (which probably means they'd avert some scheme by Savage that's gonna wipe out the whole Society.) It'd be the perfect way to setup for a potential JSA stand-alone outing, AND act as a shortcut to making the DCCU retroactively that much richer. I'm telling ya, it's pure DC.
There would be other considerations; Wonder Woman's solo film is looking to be period set also, so presumably she'd
already be there fighting Nazis. They'd probably just contrive a reason for her to
not time travel, because why would we need two Dianas when this thing would be bursting with colorful characters already?
That's the other thing, one wonders how many Leaguers should be going back when this film would be a 2.5-hour backdoor pilot for a JSA franchise. Still, I think a roster of similar size to the original 3-part episode would be plenty, especially given that was only 1 hour all cut together.
And oh yeah, following the original framing of that 3-parter, they might as well have Ben Affleck direct the thing, since Batman's only there at the beginning and the end.
Did I miss anything?