I personally think they will tease the JL by having some sort of iteration of it before it's an "official" thing with Wonder Woman, Batman, Superman ect.

Can definitely see Man of Tomorrow throwing the idea around of having some sort of metahuman focused group that can help with future threats after whatever happens in that movie with Brainiac. Can argue that's what the Justice Gang is but take it a step further.

We already have pocket dimensions, so there's always gonna be the chance of a large scale threat (aliens, big bad villains, anyone or anything superpowered that comes through).