A character who never takes anything seriously gets two nations to go to war because of a joke, and is tasked with stopping or at least reducing the death toll in said war.
An inventor makes an entirely new type of weapon planned to revolutionize warfare. When he’s killed halfway through developing the prototype, a bunch of people fight a war over who gets said revolutionary weapon before it’s even finished.
In a world where electricity was invented early, people figure out ways to work electricity into weaponry before even flintlock guns exist. So, maybe not laser swords, but possibly blades that work kinda like electric fences.
A hero and a villain each have followers. The followers all befriend each other, but the hero and villain still fight because of “destiny.”
A tree vampire who sucks sap, not blood.
People who die turn into different types of mystical creatures depending on where they die — they become merfolk if they die at sea, dwarves if they die underground, dragons if they die in a pile of treasure, etc.
Magic that only works when the user is in total darkness. Closing their eyes also works.
A species that can die of blood loss by losing a single drop of blood.
A world in which nations at war don’t send their people to murder each other, but instead have a lawsuit contest to see which nation was home to the most ridiculous, unethical, frivolous, or otherwise stupid lawsuits. The offending country loses.
A berry or some other natural food that is poisonous to men, but not women. (Or the other way around, if you prefer.)