An immortality system where people can only find out if they’re immortal or not when they die for the first time–however, this immortality only lasts until old age, when the person dies of natural causes. So theoretically, it’s possible (and extremely probable) for someone to go through their entire lives not knowing about their immortality.
Also, what if someone tried to create a race of only immortals by rounding everyone up and killing them, letting the ones who stood back up reproduce and make more immortals (assuming immortality was genetic).
A society of people who make every attempt to look as similar to each other as possible. Everyone wears the same clothes, styles their hair the same, grows their beards the same length, and exercises just enough to look exactly like everyone else. People are differentiated only by their personalities, age, and facial structure. (Men and women could each follow different models, possibly; that way, women wouldn’t have to grow beards.)
Build a simple fantasy universe with elves and orcs and stuff. Now advance the timeline by 2000-3000 years so you essentially have all those races, the magic system, and a history of evil overlords and prophesied heroes in a sci-fi setting. Then tell the story.
A magic system in which “magic” is merely a language. Each word spoken becomes true within the limitations of physics. For example, a fireball spell would translate into English as, “A bit of the fire from the pit over there flies out and smacks my enemy in the face.”
Ideals have sounds–overtones that play in societies that hold them.
Dimension-hopping titans randomly pop into existence on various world and essentially destroy everything if the inhabitants aren’t equipped well enough to handle them.
A murder mystery in which no one actually dies.
The most primitive society in the world manages to advance enough to take over a large continent because the people living there decided they were oppressing the natives too much and gave them all the land they asked for.
A world in which absolutely nothing dies just from being stabbed or shot. Each animal, sentient or not, has a few obscure weaknesses that people need to exploit to kill. Even people have magic shields that need to be disintegrated with, I dunno, cabbage.
Witch hunts against elves lead to them nearly going extinct. Now, there are only a few left in the world.
Your protagonist is not an elf. Your protagonist is actually the one leading the witch hunts.