A reward could be added for people who have highscores, such as a 20% bonus in the dungeon where the person has the highscore, or a number of Dungeon Crests relative to the number of times a person is listed in the highscore.
Another buff that could be given to dungeons is to EXP. Since the limit of 1 completion per day, monsters could give experience in addition to the dungeon's experience, or the final experience could be increased.