Hero or villain.

I would like to see a hero or villain whose powers are determined by the doubts of people. He or she would be able to do anything that people were actively doubting he or she could do. So the only way to accomplish things would be to convince people that he or she could not, to start small and work up, doing things secretly and failing at things publicly.

Maybe the best use of this is a video game where the use plays this person as a supervillain. The player would have to balance wins and losses to get better.