Avengers: Age of Ultron is widely regarded as the weakest of the four films where Earth’s Mightiest Heroes teamed up to save the universe, and while James Spader’s performance was as solid as you’d expect from such a talented actor, Joss Whedon’s script didn’t come anywhere close to positioning the villain as the terrifying and indomitable threat he’s often depicted as in the comic books.
There was talk that Spader would reprise the role in WandaVision after he was listed in the show’s IMDb credits, but that turned out to be inaccurate, and we were left with a single name-drop instead. However, Ultron will return in upcoming Disney Plus animated series What If…?, although The Blacklist star hasn’t been announced as part of the sprawling voice cast.
It’s not been made entirely clear whether or not What If…? operates as official multiversal canon, but a new theory claims it could lead to Ultron making a grand return to one of the many impending live-action timelines to launch an invasion, after several sequences in the trailer hinted the nefarious AI could show up in a number of episodes.
Were that to carry over into the movies, then there’s plenty of comic book stories that could be mined for inspiration, whether it be the actual Age of Ultron arc where he’d taken over the world, an Avengers run where Kang the Conqueror invaded a timeline that saw Ultron as the ruler of all creation, causing the time traveling warlord to constantly try and defeat the android by showing up across a multitude of realities, but failing to do so and almost destroying the multiverse as a result. It sounds like a long shot, but it’s also entirely plausible following the Season 1 finale of Loki.