Write an entire story with a compelling conflict using man vs. nature. Plant decoy antagonists here and there who turn out to be pretty decent people, and let the theme be along the lines of “there’s not always someone to fight.”
A world in which the roles of science and magic are essentially reversed: science makes stuff work, people joke and laugh about science; scientists perform “science tricks” such as making small volcanoes erupt.
While traveling with his wife, your protagonist realizes that saving beautiful princesses might become a bit of an issue. So he trains his son and daughter to be heroes as proxies for him.
Your protagonist goes through the entire story with a single friend who s/he confides in. In the end, it turns out the friend is the protagonist’s hand and the protagonist just has multiple personality disorder.
Your protagonist’s primary weapon isn’t a sword or a spear, but a grappling hook. It’s entirely possible to fight with a grappling hook (it’s an arrow with a chain or rope attached, essentially), but the most fun bit would be swinging around like a prophesied Tarzan.
People can teleport very short distances using half their current energy.
A setting in which sentient dogs and cats keep sentient humans as pets.
To defeat the antagonist, the protagonist must learn a new language and use it to out-debate the antagonist in his/her native tongue.
Set a fantasy story in medieval Europe, give the setting dragons and kings and queens and such — and then switch the story over to some other country (e.g. China, Vietnam, Russia) while keeping the generic European thing as an actual setting your characters reference sometimes.
Your protagonist’s parents find out s/he’s the prophesied hero and decide that, to completely screw with the prophecy, they should make the kid evil right from the start. Turns out, the kid becomes the evil overlord instead and the parents both try and fulfill the prophecy in the kid’s stead once s/he grows up and they’re in their 40s and 50s.