Doctor Strange co-star Rachel McAdams will not return for the film's 2021 sequel Doctor Strange in the Multiverse of Madness, according to Variety. This unconfirmed report was slipped into the larger story of Marvel Studios courting Sam Raimi as replacement director for the film, replacing the exiting Scott Derrickson.
The Hollywood trade doesn't give a reason for this, as back in 2018 it reported she was "expected" to return.
As far as who is announced to be in the film, both titular star Benedict Cumberbatch and Elizabeth Olsen (coming off of WandaVision) are confirmed.
Marvel is reportedly aiming to begin production on Doctor Strange in the Multiverse of Madness this May, to turn around the complete film in one year for its May 7, 2021 release date.