From what we’ve seen so far, Mr. and Mrs. Heart are the average couple next door that Wanda and Vision befriend and invite over for dinner. However, their presence in their home quickly turns somewhat hostile, as they begin rattling off questions at them in an interrogating manner: asking about when they got married, where they moved into the neighborhood from, and why they haven’t had any children yet. As posited by Redditor u/LisandroFM7, it could be that they’re not simply nosy, but are actually trying to further their own secret agenda, in disguise.
The Reddit user writes, “Maybe Mrs. Heart is Monica Rambeau using her Appearance Alteration Power??? And Mr. Heart is a Sword Agent?” As we know, Teyonah Parris will appear in WandaVision as Rambeau — who made her MCU introduction as a child in 2019’s Captain Marvel, albeit without appearance manipulation abilities — and the organization S.W.O.R.D. will play a major role in the franchise going forward. Though their specific motives remain a mystery, their presence could plausibly connect to Wanda’s personal emotional struggles following Avengers: Endgame.
Continuing, u/LisandroFM7 says “Maybe they want Wanda to realize that this world is not real and she is creating it.” At this point, Vision’s death at the hands of Thanos (Josh Brolin), as depicted in Avengers: Infinity War, is definitive. Wanda has yet to process it, therefore, it’s possible that to cope, she has created another world in her mind to escape the harsh reality she’s living in. For that reason, perhaps Rambeau and S.W.O.R.D. need her to “wake up” from her delusion and use the Hearts as a front to enter her fragile mind.
Bearing in mind what’s known about WandaVision‘s story, this theory doesn’t sound too far-fetched. At any rate, we’ll just have to wait and see about its validity when the show hits Disney+ on January 15, 2021.