Spoilers: (Highlight to reveal)
With Brainiac Superman leading an army of Daxamites, Kryptonians and Czarnians, which can match the Lantern Corps, which heavy hitting races or individuals do you think we will see in Injustice 3?

Spoilers: (Highlight to reveal)
Lobo motivated by revenge, fights against the Czarnians or he could be released from Darkseid's torment to lead the war against the Regime.

Surely the rest of the Corps would react to the incoming threat: Larfleeze makes a deal with the remaining Guardians to claim some of Superman's "possessions" as spoils of war. The Sinestro Corps could have Mongul as an ally.

Green and White Martians could be the first victims of the Regime as they begin to conquer the universe starting with Mars; that way Martian Manhunter won't be neglected by DC.

Atlantis will fall in line with the Regime but Aqualad could go either way.

Superman hacks the Metal Men, Amazo and Red Tornado adding them to his neural network, since every game so far has had hacking scenes: the 1st being Evil Cyborg hacking Good Cyborg and the 2nd was the Brother Eye scene.

Despero did die in the comics but he could always revive himself and attack the regime with his army.

Flash might gather up speedsters to change the past with the help of the time traveler Booster Gold.