Namor is arguably the biggest character in Marvel Comics who's never really come close to appearing in live-action. The King of Marvel's Atlantis dates all the way back to the days of Timely Comics, having made his debut way back in 1939.
From what we can gather, the Sub-Mariner's screen rights situation resembles that of the Hulk: it's complicated, and it's all Universal's fault. While Marvel does appear to have the rights to the character to a certain extent, it's believed that Universal still holds sway over distribution. The latter studio has made multiple attempts at getting a film adaptation off the ground since they first optioned Namor back in 2001, though none have progressed past the scriptwriting phase.
About a year and a half ago, Marvel’s Chief Creative Officer Joe Quesada stated that he believed that the rights to Namor were strictly back with Marvel Studios. That said, Quesada was proven wrong when he made a similar statement about that very same character a few years prior, and he didn't sound particularly sure of himself the second time around, either. It's very possible that Namor could be among the 20 MCU movies planned to follow Avengers 4, but until someone like MCU head Kevin Feige confirms it, there's no way to know for sure.
And you thought the other characters listed here have confusing rights issues. Truth be told, we still don't have a concrete understanding of which of Spider-Man's supporting characters and villains the MCU can or can't use under the shared deal made with Sony back in 2015. The only Spidey characters we know for sure that Marvel has the rights to are the folks that appeared in Spider-Man: Homecoming.
Here's what we do know: Marvel can use Tom Holland's Peter Parker five times (in Civil War, Homecoming and its sequel, and Avengers 3 and 4) before they have to renegotiate a new agreement with Sony. Sony, meanwhile, is developing its own Marvel universe independent of the MCU, starting with the Tom Hardy-starring Venom movie that releases later this year. Where the rights land for the web-slinger's rogues gallery (Green Goblin, Doc Ock, etc.) and allies (Gwen Stacy, Jessica Drew, etc.) was never made entirely clear, but it was believed that the two companies would be finding some way to divvy up those characters and distribute them amongst their respective cinematic universes. Marvel may not have secured the full rights to Spider-Man's world, but hey, Peter Parker's in the MCU, so everyone's happy. We thought all of that was relatively set in stone, but recent bombshell reports suggest otherwise.
Right. So, if Holland pops into this fall's Venom movie, then we can safely toss everything we thought we knew about the Marvel/Sony deal out the window. Dogs and cats living together, mass hysteria, that sort of thing. If these reports are true, and Holland is indeed making an appearance in Venom, then continuity laws dictate that Eddie Brock's solo movie takes place within the MCU. Marvel Studios, then, presumably has access to the full Spider-verse. If these reports are in fact false...well, we still don't really know which Spidey characters Marvel can use and which they can't. It's a pretty bizarre situation honestly, but we'll keep you updated as we learn more.