Wanda Maximoff's sitcom world, and once the villain's true identity was unveiled, her transformation into Agatha Harkness unleashed a whole new persona full of dark charm and sparkling evil wit.
Though attentions have since shifted to Gods of Mischief and the Sacred Timelines, questions of Agatha Harkness' return have barely ceased since WandaVision's final episode aired. Predictably, Hahn has toed the company line, expressing interest in a return but refusing to confirm where or when that might happen. Fortunately, Kevin Feige has been more forthcoming, recently claiming that Agatha Harkness' next MCU gig "can't come soon enough." Such a definitive statement from the Marvel Studios camp is rare indeed, which gives some idea of how highly Agatha is thought of among the MCU's creative team.
WandaVision's finale deliberately left the door ajar for Falcon & The Winter Soldier's Flag-Smashers, for instance, are not.
As you'd expect, Marvel isn't simply trying to shoehorn Agatha Harkness back into the franchise without a proper plan, and Hahn's return actually serves a much larger Phase 4 narrative. It's rapidly becoming apparent that a supervillain team à la the Dark Avengers or Thunderbolts is coming. Falcon & The Winter Soldier introduced Contessa Valentina Allegra de la Fontaine (Val to her pals) as the evil Nick Fury, recruiting various sketchy baddies for some unknown project. Her first recruit was unhinged Captain America knockoff John Walker, otherwise known as US Agent, and the series also brought Baron Zemo back into the fold. Abomination, Captain America/US Agent, etc.), so it makes perfect sense for Agatha Harkness to reemerge as the evil mirror of Scarlet Witch.
Another important factor behind Kathryn Hahn's return is the recent announcement confirming WandaVision will not receive a second season. Being forcibly trapped within Agnes served as a fitting fate after Agatha meddled in Wanda's affairs, but is hardly a satisfying place to end her MCU story permanently. Without WandaVision season 2 to continue Agatha's tale, it became inevitable that Harkness would cross into the wider MCU, not just to enact sweet revenge upon Scarlet Witch, but also to please fans who can't wait to have that tune stuck in their heads all over again.
