You could have them start in some kind of limbo, primordial chaos, or young cosmos, that can easily be shaped by their will. However, anything they create takes on a life of its own, and is not so easily unmade.

For example, they can decide the world will have two suns, and so it does. Extinguishing the suns, now, that's a different matter. And later on, they might witness the beginning of a war between two races of fire elementals.

Eventually, the world will start rejecting new additions, effectively depowering the characters.