In an interview with IGN, Kevin Feige cleared up the rights questions for Namor:
IGN: Finally, there were rumours circulating awhile back that Universal and Legendary Pictures were developing a Sub-Mariner movie, but there was also some confusion about who holds the rights to the character. So could Legendary make that movie at this point?
IGN: So if a film were to happen it would be you guys making it?
Feige: Yes, but itâ€™s slightly more complicated than that. Letâ€™s put it this way â€“ there are entanglements that make it less easy. There are older contracts that still involve other parties that mean we need to work things out before we move forward on it. As opposed to an Iron Man or any of the Avengers or any of the other Marvel characters where we could just put them in.
With strong rumors of a New Avengers movie to happen after Phase 4 (long after Avengers 3), maybe Marvel is working towards introducing Namor at some point? We know they have things planned out through 2028, so it’s definitely a possibility.