It seems that one of Marvel's first superheroes could make an appearance in 'Doctor Strange in the Multiverse of Madness', but was ultimately rejected for one important reason, according to the film's screenwriter, Michael Waldron.
The character in question is Namor the Submariner himself, who appeared in the comics back in 1939. Namor has a lot in common with DC's Aquaman, including his underwater abilities and the fact that he is heir to the throne of Atlantis, so Marvel's decision not to introduce this character until the phase 4 is understandable. Namor was one of the original members of the Illuminati team, so many fans expected him to finally make his live-action debut in 'Doctor Strange 2'. But Feige and Co. dropped the idea, and Waldron has a pretty interesting reason why.
"[We] talked about him, because he's certainly an original member of the Illuminati. But I think Marvel has other plans for him in the MCU. And so, he didn't make his way in this particular movie," Waldron told Variety.
Yep, it seems that the wildest fans' theories now have something to back them up. One of the most persistent rumors claimed that Namor would appear in the upcoming film Black Panther: Wakanda Forever, with Mexican actor Tenoch Huerta playing the mighty underwater hero. Of course, these are just rumors, but the fact that Waldron remains in the dark, despite his high position in the Marvel hierarchy, shows fans that something big is in store for Namor.