Captain America: The Winter Soldier may have tacitly teased Atlantis in the MCU way back in 2014. One major Marvel character yet to be introduced into the MCU is Namor the Sub-Mariner, the House of Ideas' answer to Aquaman (who actually predates Arthur Curry by two years). But that's not for lack of teasing.
Namor and Atlantis were first alluded to in 2010's Iron Man 2, which had a SHIELD map with a marker over the Atlantic ocean (this was alongside the first canon reference to Black Panther). More recently, it's widely accepted that Okoye's mention of underwater earthquakes off the African coast in Avengers: Endgame was foreshadowing a future Atlantean uprising. But that's not the only set up for Namor.
Captain America: The Winter Soldier opened with Steve Rogers being called up to infiltrate SHIELD ship the Lemurian Star, which had been overtaken by French terrorists led by Batroc (actually under the command of Nick Fury, who was spying on his own organization). There's a lot of comic references here - Batroc the Leaper is a striking Cap villain - but what we're interested in the namesake of the ship: Lemuria.
In the 1800s, Lemuria was a theorized sunken continent in the Indian Ocean believed to connect Australia, Madagascar and India. It has long since been disproven by an increased understanding of tectonic plates; the theory was rooted in explaining how three different modern continents could share so much genealogy. But while it's a footnote in geographical history, Marvel comics took the idea and ran with it. In the 616-universe, Lemuria was an early continent ruled by Deviants that sank under the sea at the same time as Atlantis in an event known as the Great Cataclysm. It would become a rival to Namor's kingdom.
The naming of a ship the Lemurian Star in 2014, then, suggests that the existence - or at least legend - of the lost land of Lemuria as a concept in the MCU. Remember, in the real world, Lemuria isn't a millennia-old myth like Atlantis; it's a disproven scientific theory and not the flavorful name for a boat. And, if Lemuria exists in the universe, then Atlantis pretty much has to come with it. None of this is a direct promise of a future Namor movie, let alone one dealing with Lemuria, but is nevertheless a deep-cut Easter egg that illuminates a possible corner of the MCU going forward.
Of course, there is a big question of when exactly Namor could enter the movies. The movie rights to the character are in a situation similar to Hulk: distribution is owned by Universal, meaning that were Marvel to make an Atlantean film, the rival studio would get first refusal on releasing it. Marvel's got around this issue with Bruce Banner 2008's The Incredible Hulk by having the Green Goliath appear in Avengers and Thor films in a supporting role. One popular theory has Namor emerging as a Black Panther villain, which was emboldened by the Avengers: Endgame tease.
For now, though, Namor fans can only imagine what the MCU's Atlantis - and, thanks to Captain America: The Winter Soldier, Lemuria - really looks like.
