Though the other three Avengers movies had Thanos playing an important part in the story (if only indirectly, as was the case for the first film), the 2015 outing of Earth’s Mightiest Heroes saw the Mad Titan sit out the action completely, only showing up in a post-credits scene that was not only narratively unrelated to the rest of the movie, but may have also been set several years after the film’s main event.
A new Reddit theory, however, attempts to fix this issue by suggesting that the titular A.I. of the second Avengers movie was actually a part of Thanos’ plan to locate and acquire the Infinity Stones. The key to this idea lies with the Mind Stone, which the tyrant himself gave to Loki in the first film. Though Thor’s brother failed in his mission to conquer Earth, the stone itself stayed on this planet and continued to cause problems for the Avengers, even playing a part in the creation of Ultron.
The Reddit user argues that Thanos may have been the brains behind this whole operation, harnessing the Mind Stone to create Earth’s next big bad. As well as influencing Ultron to have an inclination towards evil, the post also suggests that the Mind Stone helped inspire the villain’s initial creation by way of Scarlet Witch’s manipulative methods. It was only after Ultron seemingly met his end before he could dispose of the Avengers and cripple civilization that Thanos finally decided, “Fine, I’ll do it myself.”
Though there are surely holes you can poke in this line of reasoning, it’s still an admirable attempt to weave Avengers: Age of Ultron into the larger multi-movie drama unfolding in the rest of the Avengers saga. I guess we’ll have to see if Thanos drops any clues to his involvement when Avengers 4 comes out on May 3rd, 2019 and brings the Mad Titan’s story to a head.