Twilight Sparkle: All of you had tough choices to make. But when you made the right one and embraced your element, it helped somepony else make the right choice too. Each of you received something from the pony whose life you helped change. I know it sounds crazy, but maybe there's something special about those objects that could lead us to the location of the keys. The chest is connected to the Tree of Harmony, the Tree is connected to the Elements, and the Elements are connected to all of us. There must be a connection! I hate to admit it, but maybe Discord was trying to be a good friend after all.