If she is Agatha Harkness, and she's the same kind of character as she is in the comics, I have a feeling she'd be a lot harder for Wanda to control than the rest of the residents of Westview.
I don't think there is a question--Agnes is AGatha HarkNESs. I didn't look for it this past episode but we've been watching every episode twice, but she always has the same broach. She shows up conveniently when she needs to.
Also, even though I know people are talking about Mephisto have we really seen onscreen evidence?
I really think that Wanda is the Big Bad for Phase IV, which would be great in the sense that Wanda is a complex character that the audience has sympathy for. Wanda and Quicksilver have always been complex in the comics--are they heroes or are they villains? Having Wanda be the Big Bad would be like Willow in Season 6 of Buffy--she's doing bad things but the audience cares about her. It would be an intriguing arc that would also be a pay off for those who have followed the movies for the past twelve years.