During an appearance on Kevin Smith's Fat Man on Batman podcast (via CBR), Marvel Chief Creative Officer Joe Quesada indicated that the rights to Namor the Sub-Mariner are back with Marvel Studios. When the host asked the comic book writer and artist directly if Namor is part of the MCU, he replied:"I can't speak for studios...As far as I know, yeah we do. It’s not at Fox, it’s not at Sony...Yeah."
As for whether or not we're going to see the character on the big screen any time soon, he would only say that, "I cannot confirm or deny, sir." If he is set to appear in the near future, Avengers: Infinity War seems like a good place for the King of Atlantis to make his presence felt, especially as Earth's Mightiest Heroes are going to need all the help they can get when the Mad Titan Thanos arrives on Earth...