The Hollywood Reporter is stating that Swinton is in talks to join Benedict Cumberbatch in the film in the role of the Ancient One, the Tibetan mystic that teaches Stephen Strange in the ways of the mystic arts. The character is portrayed in the comics as a old, Asian man, exemplified in the artwork below:
Swinton is neither ancient, Asian nor male, so this casting might cause comic fans, notorious for not receiving any change from the comics well (see the files Jordan, Michael B; Storm, Johnny; and Four, Fantastic), to begin protesting. However, Swinton seems to be very genre friendly. Arguably her biggest role was in The Chronicles of Narnia series, and if she signs on here, it would mark her third comic book film (after Constantine and Snowpiercer). And to add a deeper level of geekiness, one of her ex-husbands shared his name with a legendary comic book writer and artist–John Byrne.
In addition, she is a great actress who would bring a lot to the role. If she does sign on, the film would automatically become 2000% better, even if she isn’t gender or nationality appropriate.