I know this is kind of out of the left field, but my thoughts about Agatha being possessed and that entity being able to suck up powers and life, like it did in her flashback, made me think about Thor's words in Age of Ultron, about the 'whirlpool that sucks in all life and at it's center... it's that', the mindstone. I know it was meant to be about Ultron and Infinity War and everything, but the situation in Wandavision seems awfully fitting. Maybe because the magic around Wanda looked so much like a whirlpool when she created the Hex and now that Agatha know how to make that magic erupt out of Wanda, she/the entity possessing her might try to snack on yo magic enough that it could destroy the universe and the only thing stopping that might be if Vision (her created Vision) is there to calm Wanda down and stabilize her enough that she doesn't feel anough grief to 'erupt'. Or to help her stop it.
I don't know, it just never seemed so fitting for Ultron or even Infinity War, because none of that was about any whirlpool sucking up life ^^