Namor is about to make his Marvel Cinematic Universe debut in Black Panther: Wakanda Forever and it seems that director Ryan Coogler has pretty much reserved the use of this character since the first entry. In an interview with Slash Film, Marvel Studios producer Nate Moore reveals that Coogler asked for the rights to use the character if he got to make a sequel.
Ryan is a huge fan of comic books, and in publishing, Namor and Atlantis often would come in conflict with Wakanda and the Black Panther. And so even as early as in post-production on the first movie, he was like, ‘If we get so lucky, could we maybe, could I just raise my hand and make sure no one uses Namor?’ And luckily, nobody was, and he had a great idea for it very early on. So we started building towards that idea, which I think is really fun.Nate Moore
It definitely was hinted in the past that he’s had the character on his mind since the first film. At one point they even tried to include a tease of his arrival in a post-credit sequence that was eventually scrapped. Marvel Studios has been interested in adapting the character for almost 18 years according to Kevin Feige but never found the right idea to bring him to the silvers screen. Luckily, it seems that Coogler‘s patience paid off and no one else had the right pitch before him.
Source: Slash Film