Who Is the Mystery Coven Member - Agatha All Along Theory

There is a big hint that Agatha didn't build the Coven she was supposed to in Episode 2 of Agatha All Along. Who should have been the person to fill that spot, and what comic character did they hint was maybe supposed getting revealed during this show?
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[@Narrabox]

Watching this video made me realise that a huge part of Marvel’s success were passionate fans speculating about the show together, based on their understanding of Marvel lore.
Not sure that they’re doing that anymore though mate. 😔