I think the guardians or the main dungeon bosses should level up and gain more scenes the more you feed them souls. Tho if you dont give the other girl's souls or time in the dungeon. Not only will they be weaker overtime but maybe even betray you! This will force you to put a more careful eye on how you use your souls over time. Perhaps a way to use souls to power up traps as well? Sorry if this has been suggested before or if its a bad idea.