I think that they should do a flashpoint movie which resets the DCEU timeline so a reboot of justice League with darkseid as the villain as it was supposed to be
It's already confirmed that it will be Darkseid, though I'd love to see the Legion of Doom, but first they need to introduce all members that will appear (so far, there's lex and deathstroke, black manta and reverse-flash could join, maybe cheetah)