What if the third Justice League Movie is like
Avengers Infinity War/Endgame.
The Heroes and Villians of Earth are fighting Darkseid's Army of New Gods and Parademons and also Grail,Darkseid's biological daughter.
Just like the final episode of Justice League Unlimited and Justice League:Darkseid War.
Legion of Doom
Green Lantern Corps
New Gods of New Genesis