Some info (not 100% entirely true)
Take it with a pinch of salt, got this from an IG post
Link: https://www.instagram.com/p/CJx3UQFjUZQ/
•Ryan Coogler was very adamant from the start not to recast T'Challa. Feige's plan was recast from the start, although it was controversial internally. They discussed Shuri taking on the mantle along with M'Baku and Nakia, and even having 2 characters being the Black Panther.
•Apparently, Marvel would change the film a bit and continue with T'Challa, but they didn't want to lose Ryan Coogler. Marvel allowed him to have M'Baku or Shuri as Black Panther.
•At one point, Coogler decided he would take M'baku, but from the beginning he was struggling to follow a different, more original direction. He brought it up again before it was too late and Feige agreed. Since November he has been remaking the script and his new character.
•Shuri and M'Baku will NOT be Black Panther, but both will be the main supporting characters in the film.
•T'Challa is not going to die. I was told that it was basically written in the script.
•The Black Panther's mantle will be revealed by an original character created by Coogler. He will be the protagonist and will be the King and the Black Panther. He was described to me as a strong, quiet, powerful young man, but also broken, with a very strong backstory.
•I was told that he was a member of the mining or river tribe. Apparently, a relative of Nakia.
•Okoye and Nakia will have smaller roles than before. They were the 3rd and 4th biggest characters in the first film.
•Doctor Doom is a secondary villain.
•Namor is the main villain and will be played by Tenoch Heurta
•Instead of Atlantis it will be called Lemuria (underwater city of the Deviants)
•It is a war between Wakanda, Latvéria and Lemuria
TBH, If this were to be true, then I'm a little disappointed that Shuri wouldn't take the mantle, but for this new lead character, it does sound interesting, pity it's not an established character. But if they were casting this new lead character then I'd go with someone like John David Washington or Bridgerton's Rege Jean Page