I'm hearing rumors that Kathryn Hahn is playing Agatha Harnkess, which is cool with me.
I only really know her from an old Spider-Man/FF crossover story I read when I was 10 or 11. In it, Franklin Richards was 'possessed' by a demon, which turned out to actually be a villain in another dimension, but the FF took him to Harkness for the 'exorcism'.
That thing scared the holy HELL out of me back then. After I read it, I couldn't even look at the cover for a couple of months. Anyways, in my still-10-year-old head, any story that involves her will add to the horror/creepy factor that Scott Derrickson originally had planned for the Doctor Strange sequel.
Since the Mouse isn't relying on the Chinese box office for this to succeed, maybe those elements will be kept in here.