I think it will have to be a threat that will force them all to come together to face it.
3 or 4 come to mind.
Obviously Darkseid.
Brainiac,
The Anti-monitor
Or maybe Trigon.
Of course Gunn may have something similar in mind to kingdom come, where the heroes are slowly replaced by more cool, violent ones, and we don't have a galactic ending threat, just earth based threats.
I hope not.
Eclipso is a pretty huge threat. Potentially bigger than Darkseid. He is powerful in an emotionally and mentally manipulative way, and can also be threatening physically because of his powers.No to either of those. We need a huge threat.