Justice League 2 would have followed the fight against Darkseid, including the villain brainwashing Superman into opposing his own friends in the Justice League. Although Warner Bros isn’t moving forward with the sequel, we actually know quite a lot about what Zack Snyder originally had planned for new movies. In the run-up to the Snyder Cut’s release, the director opened up in a number of interviews about what the fight against Darkseid would’ve looked like.
Snyder told Inverse that Darkseid would’ve killed Lois Lane to break Superman’s will, brainwashing him under the all-powerful Anti-Life Equation – which is a tool that would let the villain enslave the galaxy. So with Superman as the main villain, a “ragtag” Justice League would try to steal another Motherbox (cosmic technology from Darkseid’s planet, Apokolips) so The Flash can travel back in time to prevent Lois’ death.