It's from the plot of Avengers 2 and Vision's MCU origins. Vision was originally being created by Ultron to create a perfect body for himself (through the combination of vibranium with organic tissue). So this What If scenario would just be that the Avengers failed to stop his body transfer as they did in the movie.
I guess I'm failing to see why Ultron would have even happened if there was no Loki trying to take over Earth in Avengers 1? Wasn't that Tony's response to what happened?
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