Since the loss of Chadwick Boseman, Marvel had to rewrite the script and give it a totally new direction, focusing more on T'Challa's sister Shuri & teasing a war with whole another civilization: The Lost Atlantis.
Beginning Of Atlantis In MCU
Marvel Studios are confirmed to not recast T'Challa to honor Chadwick, instead will focus more on exploring more characters, civilization beyond Wakanda, and also Introduce A New Superhero as The New Age Iron Man.
New Civilization New Superhero
IronHeart Shall Be The Name
Dominique Thorne will play Riri Williams, aka The Ironheart. Se is the next Ironman who built her own suit with Tony's blessings as per the comic books. Ironheart will be a Disney+ series where she's a genius inventor who creates the most advanced suit of armor since Iron Man.
Iron Heart A Wakandan Lady?
Her origin with Iron Man will probably be avoided since Robert Downey's no longer in contract, but we can assume she could be a Wakandan and used Vibranium in her suit, as we already know Wakanda has genius people & genius tech.
Black Panther But No T'Challa
Leaks are heavy that we will see the hybrid Atlantean Prince: Namor The Submariner along with the Atlanteans in a war with Wakanda. Only this could save the movie from failing due to the Marvel's decision of not recasting T'Challa, and what good is Black Panther without T'challa.
But Why Is Atlanta Upset?
The groundwork for Atlantis & Namor has been laid for years in MCU. Iron Man 2 features a S.H.I.E.L.D logo on a map over the ocean. Avengers: Endgame featured a reference to an underwater earthquake, and most recently, the event of Eternals featured the Celestial Tiamut emerging from the Indian Ocean, which likely would have caused a major disturbance to Atlantis.
Dr. Strange 2 may also tease Namor's existence and could possibly tease 'The Defenders' also, which includes both Namor and Dr. Strange, Silver Surfer, and the Hulk in the team, As we see Dr. Strange fighting Gargantos - A minion of the sorcerer Naga. Naga is one of Namor's biggest foes.
New Avengers Called 'Defenders'