Idea: the hero goes up to fight the demon king/lord or whatever, but as soon as the demon king dies the hero "becomes" the demon king.
Think of it like the love, death & robots: the witness episode.
The moment the demon king dies a new hero "appears." Then that hero rushes in to kill the old hero. You could make it so that the new hero thinks that the old demon king that died was a beloved companion.
The best part of this is almost a gladiatorial style. It's like a perpetual shonen fight (think one piece fights on steroids.)
Shake up the formula with other conflicts if it's gonna be a long term series tho. Maybe after killing the demon king and the following hero. The new "demon king" gets prep time for the next hero.
You could make it so that when you become the demon king you gain a "system" that forces you to act evil. (I'm just putting that out there as an option.)