Here's an app-idea hard to top people, it literally saves lives if put into mass use. A simple button on the app allows you to send out an alert to one of the 12,000 registered first aid people who have the app, and the person who is closet to your location can respond with life-saving first aid, pronto.
Will it win in Cannes? It's got a shot. Speaking of, I bet you one that it will.