Namor the Sub-Mariner is undeniably one of the oldest Marvel characters in the entire MCU—dating all the way back to the late 1930s. Even so, he has yet to make an appearance in the first three phases of the MCU—but it seems that is all about to change.
For years, the rumor mills have been cranking out speculations that Namor, the water-dwelling anti-hero, would soon appear in either Doctor Strange 2 or even Black Panther 2, now being officially referred to as Black Panther 2: Wakanda Forever.
The consensus is that if Namor were to appear in one of the two films, chances are leaning towards Black Panther 2. Simply because Doctor Strange 2 is already pretty well packed with several MCU characters.

Image: QuirkyByte
However, the most recent rumors state that Namor will indeed appear in Black Panther 2—and the somewhat surprising part is that he is to be the film’s main villain. It seems that it was always planned that he would serve as the antagonist of the film. Although, some restructuring of the plot was needed with the untimely passing of the original Black Panther, Chadwick Boseman.
The initial plotline of the sequel is said to have revolved around a clash between two rulers—Wakanda’s monarch (T’Challa) and Atlantis’ ruler (Namor.)
Now, the storyline seems to be that, with the character of T’Challa written out, Namor sees a prime opening to conquer Wakanda, while the nation is mourning the loss of their most beloved leader. There is also a rumor that a massive battle between Wakanda and Atlantis will take place as well.
Although Namor has served as a villain and a hero, and everything in between, over the decades, he began as a bad guy and it seems to only make sense that he be introduced to the theatrical MCU as such.