A magical crown that grants its wearer nearly unlimited knowledge, but also takes away a sense at random.
The hero and antagonist each have fortune-teller friends who are constantly trying to get one foot over the other by essentially playing their respective hero like a game character, telling them what to do and how to do it at every turn.
A planet that actually has a burning fuse extending into space. Its inhabitants don’t realize what it is until the end of the fuse gets close enough that they can see it.
A new “life tax” imposed on every man, woman, child, and animal in a city-state. Anyone who doesn’t pay for their life is killed.
To make this more interesting, make it the good guys who impose this tax.
Indentured heroes who are forced by “prophets” to save the world with the power of love or whatever despite what they actually believe or want to do. They have to save the world even if the world doesn’t need any saving.
A person who is thought to be immortal, but in reality can just take a thousand hits before he dies.
Food that grants magical powers only after its nutrients have been absorbed; mages would have to eat this food several hours before a battle, and would be absolutely powerless otherwise.
A pair of languages that use all the same words and inflections and such, but in entirely different contexts. For example, “hello” in one of these languages could mean “I kidnapped your dog” in the other.
A system wherein extremely influential people become gods upon death.
A species possessing none of the basic five senses, but that is entirely sentient.