GODDAMMIT, I might actually have to write it now! Maybe not for xf_santa, since I don't know if I could finish that kind of fic in a week, but I want to at least try. I've actually got, um, three sentences? And it would be so easy to explain the hallucinations as latent fears brought to light -- like, Scully being uncertain of where she stands with Mulder re: Fowley and whatnot, and that could translate into her hallucinating Mulder was hiding the brain disease. Aaaand jeez, I feel like William would have an impact even if he were a hallucination? Losing him probably wouldn't hurt any less because he was a hallucination -- it could even hurt more BECAUSE he was a hallucination. And, like you said, to jump from the S8-9/IWTB level of intimacy to the end of season 6. It could be interesting.

WOW I AM SUCH AN OVERTHINKER. I'm totally going to have to try to write this now. Aaah.