Okay, so about my earlier post talking about how Will being possessed by the MF looked similar to Chrissy, Max, and looking back now, even Nancy while being attacked by Vecna. I think I have a solid thought/parallel whatever you want to call it here? (Please forgive the picture quality; Netflix hates me.)
We see the outsider perspective, (Joyce & the party // Eddie // Lucas, Steve, & Dustin // Steve.):
And the inside perspective.
(Will & the MF // Chrissy & Vecna // Max & Vecna // Nancy (though Nancy's case is different because Vecna is showing her his past.)):
So maybe this is a dumb theory; but what if Vecna was the cause of Will's hallucinations in the beginning of season 2 and like, what if he was the reason Will was possessed by the Mind Flayer? If not; well I think this is just an interesting parallel.