According to Variety, Marvel is developing one around the character of Agatha Harkness, played in WandaVision by Kathryn Hahn.
During much of the season, Hahn’s Agatha was Wanda and Vision’s cheerful if overbearing neighbor.
But she was very much a supporting character — as is the Agatha Harkness of comics whenever she’s turned up to assist the Fantastic Four or the Avengers.