Heroic Hollywood is reporting that Rachel McAdams, currently vaping up a storm on True Detective season 2, has been offered the female lead in Doctor Strange. Unfortunately the site doesn't seem to have info on just who that lead is.
If we're taking guesses, though, I would say it's gotta be Clea, Strange's girlfriend, student and sorcerer from the Dark Dimension. Clea is the most famous and standardized of Strange's love interests and companions, and her story - she's the niece of the Dread Dormammu, and she eventually becomes ruler of the Dark Dimension - makes her more clearly a modern-style female action hero who can hold her own alongside Strange.
There is another possible character, a very minor one named Morgana Blessing, a writer who published thinly-fictionalized novels about Stephen Strange's life. She has no powers and she doesn't really do much, aside from driving Clea away from jealousy and eventually sleeping with Strange's vampire brother (these comics were weird). This sort of a character makes sense if you're trying to give the audience an entry-level human perspective into Strange's life and shenanigans, but the Phase Three Marvel movies feel too smart to have the female lead be someone incapable of standing toe to toe with the hero. I doubt we'll see too many straight-forward Jane Foster love interests in the near future.
And besides, Clea is the classical supporting character, and I think that Marvel is going fairly classical with this stuff. Plus she's an entry into future stories and, who knows, a spin-off of her own. Of course I can't imagine many people have a clue who Morgana Blessing is, so maybe that blankess gives Marvel the room to play around with the character and do bold, different things.