The lineup for Phase 3 of the Marvel Cinematic Universe (MCU) is already filled and on a tight schedule, but Phase 4 might have a perfect slot in which Marvel Studios could introduce everyone's favorite aqua Marvel hero, Namor The Sub-Mariner. While Kevin Feige has confirmed the Spider-Man: Homecoming sequel will start off Phase 4, and James Gunn's Guardians Of The Galaxy Vol. 3 will take place somewhere in there, Namor would be a great addition to the #MCU.
The question is though: How will Marvel Studios introduce this iconic character with an explanatory origin story for today's audience? The answer might have already been revealed to us in the MCU already. With that being said, here are three ways Namor could be introduced to the MCU.
1. Terrigenous Infection
In Agents Of S.H.I.E.L.D., Daisy Johnson stopped her mother from stealing an entire case of Terrigen Crystals by using her earthquake abilities to push the quinjet, which housed the crystals, into the ocean. The Obelisk metal within the crystals liquified and spread through the ocean, infecting fish, plant life, and spread through ocean currents. Eventually, the Terrigen was so widespread, that it was in fish oil and even people's food. For those of you unaware, a Terrigen Crystal can transform a human with Kree DNA into an Inhuman. If the subject doesn't contain Kree DNA, the Terrigen will turn them to stone.
Multiple people were killed, but those who survived turned into Inhumans, which caused the current "rise of Inhumans" in the MCU. However, many were unaware and hid from the world. If Namor gained the ability to breath underwater, the ocean would be a perfect place to hide from agencies such as HYDRA, S.H.I.E.L.D. and the US Government, and would explain why we have seen nothing of him so far in the MCU.
This would be a great and easy way for Marvel Studios to introduce the hero without having to come up with even more complicated origin stories, and would also follow source material, as #Namor was an Inhuman in the comics. Honestly, Marvel Studios could easily pull off a Namor movie, and if not, introducing him in Agents Of S.H.I.E.L.D. or Inhumans would be fine as well.
The map above is evidence that Namor could have been infected, showing that the Terrigenic infection has gone global, and there's no telling who could have gotten infected.
2. Namor The Winter Soldier
Another possible theory is that Namor is a product of HYDRA experimentations. HYDRA is known for its human experiments, as seen in Agents Of S.H.I.E.L.D. with Daisy Johnson's mom and Captain America: Civil War with Bucky Barnes and the other Winter Soldiers. Could Namor be a product of the Super Soldier Serum with other enhanced abilities that allow him to attack HYDRA enemies from the water as well?
I wouldn't be surprised, as almost every initiative HYDRA has ever implemented has been about making soldiers with enhanced abilities, or using those who already have enhanced abilities. But wouldn't he be working for HYDRA? Well, if you remember from Captain America: Civil War (CW), some patients such as Josef rebelled against HYDRA and used their abilities against them. It would be possible for Namor to have escaped. Plus, the book containing the words to control the Winter Soldiers was destroyed at the end of Captain America: CW, so it is possible.
If a possible movie did explore this, it would explain the strange introduction of more Winter Soldiers in the film that didn't mean anything to the plot, considering most of them were killed by Zemo and put in glass vaults.
A very usual explanation for those with inhuman abilities is simply that they are Asgardian. It could be possible for someone such as Namor to possess such abilities if he weren't born a mortal of Midgard at all. It's also good to note that Asgard is completely surrounded by water, and rivers flow through it as well, and would provide a great habitat for Namor to live in.
It would make sense, as he possesses a god-like staff and he had feathers on his ankles that are similar to the feather design on Thor's helmet in Thor. If so, a Namor movie would be great way to explore more of Asgard and the other nine realms.