Mephisto Theory (Theory One)
Wanda and Vision are in an alternate reality, with no memory of their previous lives. Vision is fake, not the real one, and was created by Mephisto to keep Wanda pacified while he conquers Earth, knowing only she can stop him.
Sitcom Theory (Theory Two)
The entire show is a limited series, a sitcom, and nothing relatively interesting happens, except Monica Rambeau showing up out of the blue. Wanda then kicks her out, and they see glimpses of other universes, and they live happily ever after.