I don't know if anyone has talked about this, but the only reason Ancient One knew that Stephan Strange would be the Sorcerer Supreme is because Kang had paved the path to the future.

If kang never did what he did then forseeing the future would have been very hard if not impossible for Ancient One. Kang was governing the sacred timeline and wasn't letting any branches to occur, so she couldn't see any other version of the future where Strange wasn't Sorcerer Supreme because that would create a brance timeline of off the sacred timeline.

Hope you all get what I'm trying to say.