The conclusion of Avengers: Endgame was a watershed moment for the Marvel Cinematic Universe. Bringing an eleven-year saga to a close, it also saw the death of Marvel Cinematic Universe progenitor Iron Man. Sacrificing himself through the use of the powerful Infinity Gauntlet, Tony Stark tragically breathed his last after saving the universe one last time.
There may be an unexpected side effect to this action, however, which would continue Iron Man's legacy in an otherwise unexpected way. The power of the Infinity Stones could either bring the MCU's next villain or even return its greatest hero in some capacity.
ULTRON INSTINCT
One possibility is that the convergence of Infinity Stone energy may have somehow imbued the Iron Man armor that Tony was wearing with sentience. This is bolstered by the fact that the armor used in Infinity War/Endgame was based on the comic book Iron Man's "Bleeding Edge" armor. In the comics, this armor arguably gave Iron Man superpowers, bonding with him somewhat like a symbiote and allowing him to store it within his very bone structure. The movie version didn't emphasize this aspect, but the resemblance to the source material's look is uncanny. This advanced level of technology would be able to be manipulated by the stone's power, giving life to the dormant machinery even after Tony's own death.
This precedent has already been shown in the creation of the Vision. This android, who was meant to be a new body for Ultron, was brought to life by the Mind Stone. He was always meant to have some form of sentience, unlike the Mark 47 armor, but the higher amounts of Infinity Stone energy that the Mark 47 armor was exposed to may have still grafted a personality to the armor. This personality could be a combination of the F.R.I.D.A.Y. AI already in the armor's systems, as well as the last vestiges of Tony's own mind. Having a part of Tony Stark coloring the sentient armor would make it very similar to the MCU version of Ultron, who was built by and modeled after Stark instead of Hank Pym. Ultron is already suspected by many fans as soon to return in some capacity in the upcoming WandaVision series with Scarlet Witch and the Vision, but this second appearance of the villain may have been set up in the climax of Endgame.
Ultron II may be made from the remnants of Stark's Mark 47 armor, and simply kept around as a benign token of gratitude for Tony's final sacrifice. From there, it would remember faint glimpses of Tony's past, namely his most evil creation, and rebuild itself into a new Ultron. Many also believe that the Marvel Cinematic Universe will begin to take a more cosmic trajectory, especially with the introduction of the Eternals. One route to go forward with this idea would be adapting Annihilation: Conquest in some form. This crossover event, which teamed the various elements of Marvel's cosmic cast of characters, also saw an immensely powerful new Ultron as its ultimate villain. An MCU adaptation could keep the general gist of the story, focus on newer cosmic characters, as well as feature introduced ones such as the Guardians of the Galaxy. In sequestering the cosmic characters off for their own exclusive crossover movie, it would also achieve the planned goal of diverging the earth-based and cosmic MCU characters more, so as to keep from building up to another massive, Endgame-sized event so soon. While divorced from the Earth, however, this story would still feature Ultron, an initially Earth-based villain, as its version of Thanos.
IRON MAN 2.0(20)
The irony of this situation is that, in a way, Iron Man, the MCU's progenitor and ultimate hero, would also in essence be its next big villain. This would especially be the case if the armor, in gaining sentience, becomes not a new version of Ultron, but Iron Man himself. One possible way to do this would be making the armor some version of the Iron Man 2020 character. This villain is an amoral relative of Tony Stark, albeit one with a somewhat more Victorian looking armor. Fittingly, he's come back into prominence this year as the Iron Man comic's big villain, so it makes a bit more sense than usual for him to be included in the movie universe.
Again, the logic of the empowered technology being infused with the last memories and moments of Tony's life could be used to explain this new Iron Man, who would act and sound like a warped version of the original. Having Robert Downey, Jr. come back to voice this literal ghost in the machine could also work. The actor is supposedly done playing the Tony Stark version of Iron Man, but voicing an Iron Man who is explicitly not Tony Stark, or at least the real one, would be a way to work around this caveat. From there, the face of the MCU would be warped into its next great nemesis, adding a new armor to the Iron Man legacy, while simultaneously destroying that legacy.