For a long time everyone has been suggesting Scarlet Witch to appear but I never really saw a natural way to do it. But with Baron Mordo hunting down Sorcerers it creates and easy situation from them to team up. I always like the idea of Wanda being able to natural tap into magic. It definitely works visually, her energy is chaotic and free flowing with Strange's is clean and precise. It would be an interesting thing to explore.
Well, just brainstorming here...
If Mordo tries to take Wanda's power he could be in for a very, very nasty surprise, if she has Chaos/Chthonic Magic. Upon discovering that, Mordo would undoubtedly seek to kill Wanda, seeing himself as doing what is necessary to save the world. The existence of Chaos Magic would undoubtedly absolutely horrify Mordo and the Kamar-Taj sorcerers, spreading paranoia in that community.
Perhaps, all this attracts Shuma-Gorath, who sends forth his servant Nightmare (he served Shuma initially) to exploit the situation, seeking to destroy the Sorcerer Supreme, Scarlet Witch, Kamar-Taj, and the Vishanti, in one insane swoop, with, without ever being seen, Chthon perhaps seeing an opportunity to corrupt Wanda, and slyly using Mordo's fear to corrupt him.