The person coming back for payback is too wide. LOL. Could be Metal Sonic, for taking away his independence when we fixed him, captain metal preparing for his revenge, any of the egg bosses, etc...
Just for this quest only, we basically dismissed Max and our actions have spoken loud against the empire, with max being a peer of Eggmans in being a conqueror, even if he doesn't want to admit it. The black arms, we took advantage of their weakness, and kicked them out, a slight against black doom. Snively, not taking him, but I'm kinda not wanting him to be the one, unless he has completely lost it and us not taking him back has driven him insane. Nick's hidden backers, us stealing the lance, and basically being the well known crashers of the auction, since we don't know who they are. Heck, we could have made an decision that one of the hidden factions took as a slight. Satan for the slight not paying him back for the slight from Conquering Storm, than taking her away before anything can be settled.
The other kings have a signifacant relationship to Eggman, since they are our peers in trying to conquer the world, thus are major obstacles. Just under Sonic, which I hardly doubt their's another who could take his place.