The upcoming Black Panther: Wakanda Forever will introduce audiences to a new Black Panther, help establish Namor in the Marvel Cinematic Universe, and close out Phase 4. But beyond all that, it might go a long way towards setting up Atlantis as a true rival to Wakanda by adapting a major event from a crossover comic book.
A shot from the trailer for Black Panther: Wakanda Forever suggests the upcoming MCU film will take cues from the Atlantis/Wakanda conflict that exploded in the Marvel Universe after Namor flooded the African nation.
The trailer for Black Panther: Wakanda Forever that was released during SDCC 2022 teased a conflict between Wakanda and the Marvel version of Atlantis -- with Namor set to come to blows with the Wakandan heroes. The trailer indicates that the two nations will clash aty various points, with one shot in particular hinting at the kind of devastation that might be awaiting the nation. At one point in the trailer, the Wakandan Throne Room can be seen being blasted with a massive tidal wave, which quickly shatters the windows overlooking the city below and flooding the space. This suggests a massive wave might be coming to the nation -- which is exactly what happened in the original Marvel Comics, causing over a decade of tensions between Wakanda and Atlantis.
During the Avengers vs. X-Men crossover, the incredibly powerful Phoenix Five (Cyclops, Emma Frost, Colossus, Magik, and Namor) found themselves imbued with cosmic power. Initially, they used their abilities to try and elevate the world around them. But as the five became more corrupted by the power they now possessed, they acted with far harsher tactics and motives. This built until Emma Frost convinced Namor to unleash his rage at the defiant Avengers by targeting one of their safe-havens -- Wakanda. Using his Phoenix powers, Namor caused an absolutely massive flood that severely damaged the nation -- and killed countless innocent citizens of the nation. The Avengers were able to defeat Namor, but the damage was done.
The fallout of this attack was felt for years within the Marvel Universe. Storm's allegiance to the X-Men at this point in the storyline (although she turned against the Phoenix Five as a result of this event) led to Black Panther annulling their marriage -- putting an end to their romance for a time. Black Panther's fury with Namor over his actions lingered throughout New Avengers when both men joined the Illuminati. Despite their goals being the same, Black Panther made it clear that he hadn't forgiven Namor for his actions -- and even left the Atlantean to die in an attempt to stop the Cabal. Atlantis and Wakanda have targeted each other frequently in the comics since then, nominally because of lingering tensions over their conflict.
The shot of the MCU Wakanda Throne Room being flooded suggests that something similar to the Avengers vs. X-Men event could befall the cinematic version of the nation. While the absence of T'Challa from the MCU following Chadwick Boseman's death means his fury with Namor won't make it to the big screen, it would not be surprising to see the next Black Panther (whoever that may be) gain that confrontational element in his stead. Atlantis going to war with Wakanda in the comics involved a truly massive attack, and if the films are replicating it then the African nation might find itself dealing with a lot of death in the near future -- and might find themselves truly going to war with the undersea kingdom.