Worldbuild a sci-fi setting with laser guns, lightsabers, spaceships, etc. Then make it fantasy. Alternatively, take a fantasy setting and make it sci-fi.
A protagonist who has sworn to be evil after a life of being good, but who makes so many mistakes that every time he tries to do something horrible, he ends up doing something good.
A time-travel system rooted in dogs. People can warp through time, but can only see through the eyes of dogs throughout history and into the future. They can’t change anything. All they can do is watch — and listen, and smell.
A group of explorers from magical society discover a village full of people who assume their magic is merely fancy technology.
Design a magic system that produces the greatest possible amount of explosions. Now give it a huge cost for looking awesome, like sucking all the oxygen out of the atmosphere.
Your protagonist is a serial killer who uses magic to commit his murders. Remember, the protagonist doesn’t necessarily have to be the good guy.
Write a ridiculously convoluted plot involving backstab after betrayal after treachery; completely ham it up, then actually bring it all to a sensible resolution.
A labyrinth full of deathtraps and treasure, often visited by borderline suicidal adventurers. Make it legitimately dangerous, not just a “test of might.”
A government imposes a new rule stating people cannot be idle for more than one hour of their waking day.
A character who, in an effort to become the most versatile person in the world, works every day to learn new languages, cultures, sciences, arts, history, combat, etc.