The clues have been there all along
Both Vision and Wanda were introduced in 2015’s Avengers: Age of Ultron, and though no one could have predicted they’d be in their own streaming show six years later, star Elizabeth Olsen does acknowledge there are several shots in that film hinting at Wanda and Vision eventually having a relationship, which pays off in WandaVision.
https://gizmodo.com/wandavisions-catchy-new-agatha-song-is-a-vintage-bop-1846327151
