Much of the fun in WandaVision comes from spotting the sitcom parallels the love-letter to TV history creates. One parallel many fans may have noticed is the last name of the series' star Elizabeth Olsen, whose sisters, Mary-Kate and Ashley, were sitcom legends in their childhood as Michelle on Full House. The two reportedly won't appear in the new Disney+ series, but that comes as a shame given just how perfectly they'd fit.
On one hand, the decision not to use the Olsen connection as a resource is understandable. After all, the show has its own focus and story it's telling. WandaVision director Matt Shakman told Variety that he didn't ask Mary-Kate and Ashley Olsen for a cameo or ask Elizabeth to do so on his behalf because the show "wasn’t about parody or homage or spoof."
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However, while Shakman provides a reasonable explanation, the more thought that goes into what an Olsen twins cameo could have done for the show the more perfect it seems. There are obvious connections, such as their familial relationship with the series' star and their titanic status in TV history the show so adoringly winks at, but even beyond the obvious, the possibilities an Olsen twin cameo provides are astronomical.
Even apart from WandaVision, the Olsen twins have not acted together on screen for over a decade and a half. Their natural charisma has been part of their charm for literally their entire lives, and it's a shame to think that they would remain in permanent retirement. If ever there was a project more appropriate to come back to the sitcom set for, it's hard to imagine doing better than WandaVision.
The connection to their sister Elizabeth would also go well beyond just a cute wink within the context of the show. Though it's still too early on to tell just what is going on, what's been clear from the first two episodes of WandaVision is the eerie sense of unreality and strange disconnect between truth and fiction. There's no better way to add to that disturbing ambiance than to insert a cameo that ties real-world events into the show's plot.
The audience is already made to feel uncomfortable by, for instance, shifts in camera setup that disrupt the comfort of a sitcom with the unease of a David Lynch flick as they watch Wanda go from suburban housewife to grieving widow. Once the audience settles into those transitions, it would be easy for the show to disrupt them once again by peeking past the curtain to remind them that Wanda is actually Elizabeth.