Kara has a connection to the Krypton backstory. Heck, you could introduce her along with a "shrunken" Argo City and Brainiac in a sequel.
Superboy's backstory would take too long to play out over the course of 1 or 2 films. if they were doing multiple films like the HP films, then maybe.
the only way I could see Superboy having a role would be if he and Bizarro were merged into one character.
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