I'm thinking Superman might end up like The Hulk in the MCU, only appears in team up movies and not get any solos. The thing at least with the hulk there's an excuse for him not being around to have his own solo adventure given the character could be anywhere in the world with the hulk having a mind of his own but with Supes he is bound to his own city and environment so don't know how they will explain away him not having his own solo movie.