I had a spiffy idea for the dungeon, if it can have at least 3 floors: What about making it like a training grounds for trans characters? (but normal could enter too)
Floor 1:
Monster or monsters at around level 20 or 30.
Double attack compared to regular monsters and double exp.
Floor 2:
Monster or monsters at around level 70
Increased attack with increased exp. (Maybe not double because they will start to hurt)
Floor 3:
Monster or monsters at around 90
Increased attack and possibly extra monster special skills and buffs. Beating them grants increased exp and better than average drops.
Something like this?