I've had versions of this idea for some time, but tonight I think I've solved the major design issues that plagued it.
The concept is that the player can turn living things into inanimate stone - statues essentially. This power is used to solve various puzzles. Examples include turning enemies into stone and using them as stepping stones or platforms to reach other areas - turning something light into heavy stone to alter the physics of the location.
Originally I'd had the idea as a magic sword, and then as trolls turning to stone in sunlight, but finally I'd realized that for full level design flexibility it'd have to be some sort of cast spell - a bounce-able sparkly projectile cast from a wand probably.
Thoughts?