Supervillains were easy. A cosmic being arriving to give them the power to reshape the world, they took it. Made the world, or more exact, the people in it their play things. Even some heroes would take up the offer in hopes of remaking the world into a better place.
But many heroes would instead wait out the clock, allowing the day to run out and the world to return to normal. For those heroes, special measures would have to be taken.
Two main options or the combination would be needed. The first involved saying the hero must use the power, and in a way which would benefit them selfishly. Even if it has some unselfish elements. Otherwise the power would be given to one of their greatest enemies. A being without morals who would use it.The second was for the cosmic being to use their powers to make the hero play along.
Or mix the two. Tell them of the chance their enemies would get the power, then negate any rejection of using the power.
This is the world where the cosmic being played a game. With a villain, a hero or a heroine... making sure people would play it as it had been designed. Someone like...