Well, this one is far from confirmed, but a rather important person is claiming that Sub-Mariner is back at Marvel Studios and can now be a part of the Marvel Cinematic Universe.
Speaking on Kevin Smith’s Fat Man on Batman podcast, Marvel’s Chief Creative Officer Joe Quesada was asked if Namor was at the studio and he replied, “Yes, as far as I know yes we do [own the character]. Yeah, it’s not at Fox, it’s not at Sony.”
Previously it had been reported that Namor was still under the ownership of Universal Studios along with The Incredible Hulk (who can be part of The Avengers but can’t have a solo movie) and Kevin Fiege has said that contract difficulties between all parties mean it wouldn’t be easy for Sub-Mariner to hang out with his MCU buddies. If Quesada is correct, it looks like all those contract issues are now fixed.
However, Quesada had previously said he misunderstood the question when it was first asked and assumed they were talking about the comics. So there is every chance that he got his facts mixed up and Namor is still at Universal.