I remember Feige getting annoyed with somebody comparing comic book Aquaman and Namor and asking if Namor was coming into the MCU since Aquaman was popular in DC's movies, and how Namor and Aquaman literally have parents who were from Atlantis. Feige's response was something about Namor/Sub-Mariner was introduced first in the comics and Aquaman was introduced later in the 1940s, and they had always planned to introduce Namor in the MCU and it had nothing to do with Momoa's success as Aquaman.
Personally, I do think they planned on bringing Namor in (or at least I'm happy they did) because at least in the 70s/80s comics I had, Black Panther and Namor were always butting heads/playing off one another and the kid in me wanted to see that in the MCU.
And at the time they were planning a lot of this out, Feige had no real access to Spidey and his villains and X-Men/Fantastic Four and their villains.
Now they have the same problem with the Thunderbolts and Suicide Squad, especially with Amanda Waller and Valentina Allegra de Fontaine giving off vibes of being the same characters at times, and John Walker/US Agent and Peacemaker, etc.
Yeah, it's a little too on the nose at times, so they have to change it up. At least now they have control over X-Men/F4 and their villains going forward.