In addition to traveling through the Upside Down and meeting random kids with powers like those possessed by Eleven, we’ve also seen characters become possessed by the power of the Mind Flayer.
This was primarily seen throughout Season 3 when Billy Hargrove became possessed after entering the Upside Down.
Throughout the season, Billy (Dacre Montgomery) kidnapped victims to help feed the Mind Flayer as the entity grew stronger.
What if the Mind Flayer didn’t die at the end of the season and took hold of Hopper when he went through the gate?