Namor

At the beginning of Avengers: Endgame, Natasha gets a report from Okoye about an earthquake under the Atlantic Ocean. In the Marvel comics, the Atlantic is the home of the underwater kingdom of Atlantis, ruled by the Marvel hero Namor the Sub-Mariner. So this may be a nod to the future introduction of the character and the kingdom of Atlantis in the Marvel Cinematic Universe.