Guardians of the Galaxy auteur James Gunn is busy writing the third instalment in his hit Marvel series, but he's running into a bit of a roadblock. He's thinking about retconning an established part of his universe for the next film. We're pretty sure we know what he wants to change -- and while it wouldn't destroy the Marvel Cinematic Universe, it does raise the question of what it means for the MCU to break its own canon.