Given each season does a misdirect I like the idea of Manheim and his Meta thug not being the real main antagonists. An idea I just got to thinking about happening would be this Earth's John Henry coming back as a villain and killing Manheim and his organization. I'm thinking a good twist would be Superman and Steel going to take down Bruno for good but finding everything wrecked and Bruno turning up dead.

Besides I'd love if he returned for revenge as Ultra-Humanite or Metallo which I think would be more appropriate like at some point the name John Corbin could come up and it could turn out to be an alias used by Irons after faking his death by surviving via extensive cybernetic implants. Besides I can see Metallo easily wiping out Manheim's operation and maybe Manheim kidnapping his sister could be what triggers him.