Last week's episode of WandaVision ended with the reveal that Agnes is indeed Agatha Harkness, and has been pulling the strings in Westview since the beginning. We don't know exactly what that means, as it's clear Wanda Maximoff has some control, but Agatha is definitely a villain (she killed Sparky!) and wants something from the Scarlet Witch.
There have been plenty of Mephisto hints, and Nightmare also remains a strong possibility, though we wouldn't be shocked if Agatha has actually been combined with the latter for her MCU debut.
Agatha revealed her sinister ulterior motives in a musical number which we now know is titled "It was _____ all along." As expected, it was written by Frozen duo Kristen Anderson-Lopez and Robert Lopez, but actually performed by Kathryn Hahn (the female voice we hear in the song).
As you may have already realised, this tune was supposed to pay homage to The Munsters, the classic sitcom which ran from 1964 to 1966. Even the title cards were nearly identical, and that plays nicely into the clearly supernatural nature of Agatha Harkness. You can watch the sequence here.
In the comics, Agatha has typically been portrayed as an ally to the Scarlet Witch, but she'll arguably make a better villain, especially if Marvel Studios does pull ideas from other characters.
What are your theories about Agatha's role in WandaVision?