i good idea would be presistant ghosts

the first time you meet each they would be playfull

the 3rd time they might get annoyed

7th time they would start getting hostile with all of them having a tier 10 form

give them neon glow each based on their personalitys and each with their own abilitys to stop us

example one might have frost where the whole area would be icey colder and even cause you to become frozen

this could also make it fun to play long term to see their progression to being more mad and evil

you could also make a ending where once you claim all of the ghost girls at their tier 10 stages they all use their t10 modes to mate us in our personal bed and they even walk around the safe house just hanging out