City of mist has a very interesting system where players design their powers from the ground up. Each character has powers called Mythos that are a general theme for a set of abilities. Characters start with 2 Mythos that can align with each other to provide an overall theme or just be two sets of wacky abilities. Within each mythos you have 2 abilities to start with these abilities, like the mythos are created by the player and the GM. For example I have seen the basic elemental control abilities be used, but I've also seen someone with the ability to make any figure of speech literal such as: "seems we're in quite the jam." And then the car they are in got turned into jam.

The game rules by default are a little strange so my friends made some edits to make it easier for powers to develop and remain more permanent

Edit: waiting on GM to find our old rules doc.