I couldn't find the post, but I've seen discussion on-line about Namor showing up in Eternals. Eternals is a possible place to show Atlantis before it sinks into the ocean in the past.
And I think there's SOMEWHAT of a consensus that Doom can actually show up in multiple movies to sort of build himself up as a larger threat. You can see multiple people speculating that on this very thread going back a couple pages.
But outside that no... I doubt there are too many people in a Spider-Man or a GotG or a Shang-Chi thread speculating that Doom or Namor would show up. He would be sort of a weird fit. Doctor Strange is a possibility though. Either could work... but Doom in particular since he's a sorcerer. Given that Strange comes first that could easily be the first place Doom shows up.
I think the more interesting thing isn't necessarily that people are necessarily wanting Doom or Namor to show up in other movies... but rather that I don't think they necessarily would be against it. If Doom did show up in Dr. Strange, I'm doubtful Dr. Strange fans would care or start fearing he would hijack the franchise. Maybe I'm wrong in that assumption, but I think it wouldn't be a big deal either way. Because it shouldn't be. Namor showing up in Shang Chi I think would be weird... but I don't think Shang Chi fans would necessarily jump to the conclusion that Shang is being replced by Namor. For whatever reason there's just this fear with a BP portion of the fan base with these characters that I don't think exists anywhere else. But maybe that's just me.