Doom deserves every bit of credit he's getting. The first thing he'd do is form an allegiance with the most powerful villians in DC and ( after researching them extensively) and slip a Doombot in to replace him as their associate while building defenses against their inevitable betrayal.
He would also research the entire hero population and (After realizing that Superman is insanely powerful) add kryptonite to his arsenal while shielding himself from being damaged by it being completely aware aware of it's radioactive effects. He's the type who would make it stronger and far more effective by manipulating it with sorcery and chemistry and probably kill Superman from a distance quickly and effectively earning more appreciation from the enemies of Superman.
He'd appear to be a strong ally to those who are power hungry and they would be unaware of the fact that a Doombot was substituted after their initial analysis of this unknown character, and carried out complex plans and ideas as Doom himself would. ( Doom would simply relay his own plans to the robot from a distance )
After completing his research on the villians Darkseid would be stripped of his omega beams by a unique invention that would transfer the power to wield them to Doom. ( through some form of trickery quite likely ) Either that or he'd create some form of "leash" that would make Darkseid his slave ( similar to the "collar" he placed on Blastarr)
After this he would continue his research and after garnering every speck of information about his supposed allies, He would destroy those he could not control.
Trickery is his game and he's quite adept at gaining strong support for a cause that he makes appear righteous. (For example destroying Darkseid for all of earths heroes to see or showing them that he has incarcerated Darkseid for his crimes in the name of justice) He's a politician remember?
Batman would see through this ploy, but stand alone and be targeted by Doom as a serious threat that needs immediate removal.
Eventually he'd rule DC's earth
Maybe he could be stopped but not if he was patient and played his cards right.
Does anyone think things could play out like this? 👿