Everything happens as usual, but the moment the big battle starts, the MCU's heroes are replaced with all of Earth's mythical sleeping kings and heroes, along with any pals/knights/armies they are alleged to have kept in their myths. Objectives are the same as the movie; everyone has all relevant information that the Avengers had about the various setpieces and macguffins. How do they do?
I realise this brings up the typical questions of what stories count for some of them, so feel free to use your imagination, just say what interpretations you're talking about.