OK just watched the final two episodes. That was a fantastic ending! This is right up there with the best of the Disney+ MCU shows. Interesting that they chose not to include any credits scenes in this show, though. That's a first since Endgame, right?
So it really was Agatha all along ... even if it was also Billy Maximoff's superhero origin story. (I notice that they never referred to him as Wiccan on the show, but his superhero costume looked comic book accurate to me.)
One thing I am wondering about, though, is why Agatha kills other witches and steals their power. In WandaVision, it seemed like she just wanted to take their power to make herself more powerful ... but in this show, the fact that she was always seek out covens of witches to kill makes me think it might be something to do with extending her life (like Mother Gothel in Tangled). Although Mrs Hart/Davis wasn't a real witch, so tricking covens with the song could just be a convenience thing - she can take multiple witches' powers at once instead of having to seek them out one at a time.
It seems pretty obvious we'll see Billy and ghost Agatha again, along with the new version of Tommy ... but what I want to know is: will they bring Wanda back and then reunite her with Vision and their boys? Vision Quest has been confirmed as another WandaVision spinoff, so it's a possibility, right? It might have to be an alternate Wanda, though (like Earth-838's Wanda from Dr Strange 2).