Superman: Man of Tomorrow is good refrence of what this film could be
James Gunn said it would focus on Superman early in his career also I think Superman: Man of Tomorrow got the right idea of having J'onn J'onzz aka Martian Manhunter as a mentor as opposed to Jor-El while expanding on the wider universe.
This clip captures Superman for the Modern era
https://www.youtube.com/watch?v=fwRKNXa_52o