If you add same events and interactions for other girls as with Yuna with slight changes + unique interactions only to them then that would be awesome.
Also it would be great if the running was more enjoyable. Perhaps you can do that by adding scenes with some enemies such as sexy psycho or add bosses?! with pretty lewd scenes. Right now running is pretty boring and just repetitive after you learn the attack patterns of the enemies and after you reach a very high amount of stamina.