WandaVision has wrapped up and still, there are discussions of Mephisto. Granted, a lot of details in the later episodes did point to the red demon such as Señor Scratchy the bunny, an alias that Mephisto used in the comics, and now, it's been revealed that it was no coincidence. Originally, that bunny was a demon.
However, this was cut due to the finale being too busy and frantic. In an interview on Kevin Smith's Fatman Beyond, director Matt Shakman stated, "They go down to get [the Darkhold], and they're reaching for the book and the rabbit hops up in front of the book and they go 'Oh, it's Señor Scratchy! He's the best!' and they reach over to pet him.
And he hisses and this whole American Werewolf in London transformation happens and the rabbit turns into this big demon, right, into her familiar, you know?" Her familiar, a demon, something more than just a bunny - is it Mephisto? Perhaps, but either way, it was cut. Still, there's the potential that Agatha's familiar is still lurking around, and perhaps that's what will free her from Wanda's punishing spell.
Not only was this a cut element from the script, an idea that never saw the light of day, but it was a fully filmed scene, albeit with unfinished VFX. This would have included Monica Rambeau and Ralph Bohner more as well, as the two would have met up with Speed and Wiccan to venture into Agatha's home to steal the Darkhold from the basement.
This could have also been a repurposed version of Agatha Harkness's son from the comics, Nicholas Scratch, a warlock. Rather than having a literal child, she could have had a pet demon, a familiar, that underwent the appearance of a rabbit. At any rate, it's unlikely that Kathryn Hahn's time in the MCU is over, so perhaps this cut plotline will be repurposed down the line. After all, Wanda stealing Vision's body was an idea toyed with in Endgame that ultimately got the axe.
Source: Fatman Beyond