I saw an interview with the head Marvel movie guy and he said they have been planting seeds for the multiverse since the beginning. He pretty much said the first was Samuel L Jackson playing Nick Fury, like there's the original comic Nick Fury in another timeline. I don't know if that's him being revisionist and acting like everything was planned, but cool if true.
Fans theorized that the kid in Iron Man 2 wearing the Iron Man mask was Peter Parker, then the MCU producer said "uh yeah, thats him now" And also fans wanted Loki to cast an illusion instead of dying at the hand of Thanos, then Classic Loki's story was exactly that. To an extent, they take the fans' ideas and theories and make some of it canon.