Sure, Avengers 5 seems a long way off with no announced release date so far, but that just leaves more time for speculation. This indefinite wait is well-spent poring through past Marvel Comics looking for characters who haven't appeared in the more than 20 films released so far. So where better to look than all the way back to Marvel Comics #1?
Namor the Sub-Mariner was one of the first superheroes introduced to the Marvel Universe, but "superhero" may not be the right word. Namor was introduced in 1939 as a "man of the deep" with the power of flight, super-speed, and super-strength. A parallel could be drawn to DC's Aquaman, as they both reside in Atlantis, can communicate with marine life, and breathe underwater, but he's more like DC antihero Black Adam. (For the record, Aquaman first appeared two years later in 1941's More Fun Comics #73.)
Over the course of decades of comic appearances, Namor fought Nazis alongside Captain America, suffered amnesia and was saved by The Fantastic Four, and was informed by Charles Xavier that he was in fact a mutant. He both fought with the Avengers and against the Avengers, with the X-Men and against the X-Men. His loyalties shifted from issue to issue.
This leaves countless opportunities for Namor to enter the Marvel Cinematic Universe, in fact, there have been plenty of rumors of him making an appearance beforehand. In Avengers: Endgame, Okoye reports there have been earthquakes under the ocean. This is dismissed pretty quickly, but it could be the tease the introduction of a certain half-Atlantean character.
One of Namor's fiercest rivals in comics is Black Panther. After all, they're both royals from hidden nations of Earth. If Namor is to appear as a villain, it could have something to do with Wakanda. However, that role could set up further appearances as a legit hero. It would be a redemption arc held in the balance, as the core of Namor's personality is his self-serving tendency.
The MCU has explored this kind of redemption before (just look at Gamora and Nebula), but a new hero who makes no qualms about only being in it for himself would add an uncertain internal tension to the super squad, which the Avengers may be lacking after Endgame.
Just in terms of versatility and vitality, Namor more than deserves a live-action appearance. Who else has appeared in so many different roles, with so many different squads, working for so many different goals? He may not be 100% a Marvel hero, but he's definitely a Marvel unsung hero.