Are we talking standalone franchise, or part of one of the established universes?
SS for the former, and Namor for the latter.
SS would work better if he had to be on his own simply because he's got an entire universe (and alternate dimensions) to explore. Even in spite of how many characters and planets would be off-limits, the writers could get very creative with his adventures by virtue of the fact that his domain is so expansive. Namor would be a good integration due to how his concerns for his people would tie into and/or clash with the issues that other heroes (be it Avengers, FF, or X-Men) have to deal with.
I think that Namor emerging from the ocean with an army of pissed off Atlanteans in the wake of Infinity War's devastation could potentially make for a decent segue into Phase 4. Seems like we'll never get to see him though, as the powers that be at all of the major studios haven't expressed interest in the character.