Could Tony Stark rebuild Ultron in order to defeat Thanos in Avengers: Endgame? Endgame's title relates to Doctor Strange's words in Avengers: Infinity War, after the sorcerer used the Time Stone to examine over 14 million possible futures, and found only one in which Thanos was ultimately defeated. It's generally presumed that he handed over the Time Stone at the precise moment necessary to ensure that future came to pass - whether through Tony Stark's survival, or Scott Lang's being trapped in the Quantum Realm.
But the "endgame" concept can actually be traced further back into the history of the Marvel Cinematic Universe, to Avengers: Age of Ultron. In that film, Tony Stark attempted to build an artificial intelligence to protect the world from alien threats. He simply couldn't believe that the Avengers would be enough to stop the threat he feared was coming. "We're the Avengers," Stark snapped. "We can bust arms dealers all the livelong day, but that up there... that's the endgame." Naturally, Stark's prediction proved correct; Thanos finally came to Earth, and the Avengers failed to stop him.
Avengers: Endgame will clearly be the story of how the Avengers ultimately deal with this crisis. They're expected to either avert or undo the snap, probably using time-travel. But Thanos is still out there, and he's unlikely to just leave them to it. The minute Thanos learns the Avengers are threatening to undo his victory, he'll come out against them with all the might of the six Infinity Stones. That means the Avengers - whose ranks have been seriously depleted - may well need something to slow Thanos down. Ultron Mk II could be the solution: an artificial intelligence running a fleet of androids.
There is actually some evidence that Marvel toyed with this idea. The Avengers: Infinity War Day One video included two pieces of concept art, neither of which actually made its way into the finished film. One - an image showing Thor and Rocket fighting side-by-side - turned out to be an alternate Thor plot that saw the God of Thunder battle the Midgard Serpent. But the more interesting piece of art showed Tony Stark stood before a series of Iron Man armors. The idea didn't fit with Avengers: Infinity War at all, and in fact the concept art has never been re-released or commented upon since by Marvel. Given Infinity War and Endgame filmed back-to-back, is it possible this art was related to a plan for the fourth Avengers film?
This is only speculation, of course, but there'd be a neat symmetry to it. Avengers: Endgame is expected to be Robert Downey Jr.'s last film in the MCU, which means this is Marvel's last chance to complete Stark's character journey. A core part of that journey is the desire for redemption. In retrospect, the creation of Ultron was Stark's greatest mistake; it led to the destruction of Sokovia, and as a result the United Nations passed the Sokovia Accords. The Avengers schismed over the Accords, meaning they were divided when Thanos came to Earth.
Ironically, Ultron - designed to be the solution for the threat of Thanos - led to the weakening of the Earth's defenses. It would be so very appropriate for that to be turned upside-down in Avengers: Endgame, which Stark successfully using Ultron Mk II to keep Thanos at bay while the Avengers put the universe to rights. We know that Endgame will revisit events from the Avengers' past, and Infinity War had a surprise villain encore in the form of Red Skull. If Avengers: Endgame does see Tony rebuilding Ultron in order to save Earth, it could be the perfect conclusion to Iron Man's arc in the MCU.