WandaVision's big Episode 5 reveal had long been in the works

CNET

"We were like, how in the world are we going to make this make logical sense?" showrunner Jac Schaeffer tells Marvel.com. "Like, how do we justify this? Because that's the thing, you can hatch a million great ideas, but to make them land, to make them be grounded, to make them feel organic to the larger story."