I wonder if WB would let them have a "one night only" pass to use Batman for this.
Yeah, they should definitely throw the DCU characters in at least for brief cameos. Swamp Thing and Stargirl should be up and running by then, in addition to Titans and Doom Patrol.
They got a year to figure this out re: contracts, scheduling, etc. Imagine if they can somehow work in all this:
Arrow, Flash, Supergirl, Legends, Black Lightning, Batwoman, Titans, Doom Patrol, Swamp Thing, Stargirl, and references and cameos to past DC TV projects like Smallville and Gotham
Then end with it all being folded into a new Earth. Shit. Be snarky as you want about budgets or cheese or whatever, but that shit would be FLAMES.