The consequences of failure where what interested me. Perhaps the King killed the queen and made the spell permanent. Damning the princes to a life as bears and a whole lot of grief. That is what interests me for a plot hook or seed.
Start with a kingdom in ruin and the king overcome with grief. The hunting of bears forbidden as his sons are now bears, the lands are over populated with the animals. Perhaps they party is attacked by a bear and they slay it and are imprisoned by the king and his small group of guards that have remained. Perhaps higher level players have some magic to undo the curse.
I could see something like a den of the brother with three paragon bears in pathfinder terms. Maybe a crumbling castle of the mad bear king.