A species whose speech is completely soundless, yet conveys meaning and intention more accurately than any spoken language.
A species capable of traveling quickly through the ground without displacing it.
A Hitler-like dictator who aims to create a flawless race (at least visually) and takes drastic steps like genocide to see that goal realized. The twist: this dictator has almost universal support. That means even if the heroes manage to take him down, they’ll still have to deal with a huge amount of people who hate them for killing their leader.
A limited magic system wherein people can choose to reset a day and do it over again, but only up to eight times.
Write a Force-like magic system where literally everything that happens is explained with a hand-wavium and everything is technically possible. In this world, since they can do everything, everyone’s main focus it to make their lives as dramatic as possible, and so they learn storytelling from a young age and essentially role-play out their entire lives.
A magic system wherein sitting with your hand on your chin increases your thinking power, making binoculars with your hands gives you super-sight, flexing your biceps gives you super-strength, and so on.
A race that completely changes form during puberty–for example, a squid can become humanoid or a humanoid shifts into a bear.
Your protagonist and antagonist have the same name and face. They are virtually indistinguishable from each other, and everything one does, the other gets credit for. So for the entire story, everyone thinks the good guy is the bad guy and the bad guy is the good guy.
Musical fighting in which everyone who fights must fight to the beat of whatever the local bard plays. This is a strictly enforced law–those who fight off-beat face jail time.
Also has a side-effect of making fights look like aggressive dances.
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).