Eleven's despair over losing to Vecna in Stranger Things season 4 is sure to make the villain even more powerful, and Will more vital, in season 5.
The previous Stranger Things monster, the Mind Flayer, took over Hawkins by possessing Billy Hargrove and forcing the residents of the town to drink various chemicals to form a type of small Upside Down army. Stranger Things season 4's Vecna, on the other hand, is taking a completely different approach that's much more intelligently sinister and all-encompassing in scope.