I don't think that the show will end with Wanda as the villain either, since that would be a pretty foul way to wrap up a story that is ultimately about Wanda's grief and how she's processing it. I do, however, think that the big superpower showdown at the end is going to be between Vision and Wanda. They teased that pretty strongly in episode 5, and the more Vision realizes what's going on, the more likely that becomes.
I am 100% on the side of thinking that there isn't a larger comic villain in the mix, or at least won't be the focus of the showdown in the last episode. If there is a Mephisto or someone else manipulating Wanda, that will be revealed as a tease at the end or post-credits. I don't expect all the loose ends to be wrapped up in this show - that's not the MCU way. I do expect that the final episode, while action packed, will focus more on the emotional fall-out with Wanda. That doesn't leave much room for the "big bad" to be revealed, given a backstory, motivations, and an explained power set in time to be emotionally relevant for what I expect to be the way this ends. That's why I think the showdown is between Wanda and Vision.
Of course, that could all change in the next episode, and if it's going to, it kind of needs to, if they're going to build to a satisfying climax.