Supergirl, just because it doesn’t make sense otherwise.
Jimmy Olsen (honestly, I’d prefer a different take because Mehcad Brooks’ version really wasn’t anything like Jimmy Olsen).
Steel definitely. Man would I love to see a good take on him, especially with some references to his history in Suicide Slum. I mean it’s hard to see this Superman ever having dealt with Suicide Slum’s issues since he doesn’t seem to really care about Smallville’s problems at the moment (that itself is one very odd thing to me — like, he grew up here and supposedly likes it here?), but it’d be nice to have Steel be a sort of reality check for Clark, as well as maybe being a way to tie Clark and Lois back to Metropolis (maybe via some other things Morgan Edge is doing there).
— as another aside, the Lex twist here is neat I guess, but it’s still Lex Luthor (or at least A Luthor) trying to kill Superman. I think it would have been more interesting if the Stranger was actually a Superman ally (like Jimmy or John Henry).
Uh, back on topic...
I think I’d also like to see some of the more minor (and unfortunately, more short lived) Superman-related heroes show up, like Guardian or even more obscure ones like Gangbuster and Thorn. Some characters who can give a little hint to Superman’s career in Metropolis.
I’d love Black Lightning to show up, both because in the comics he was originally from Metropolis and also because he’s the only other Arrowverse hero with teenaged/adult children.
Martian Manhunter, for the same reasons as with Supergirl (he’s also another alien/mature character).
In general it’s a bummer Supergirl and Black Lightning are ending. I think there’s more crossover potential there than with Flash or Batwoman at the moment.
And finally, Batman. And Wonder Woman. Because, as Bruce Wayne once said, sometimes people deserve to have their faith rewarded (what a great ending, even 13 years on).