Had an idea at the end of the game when the MC is revisiting each gens area he could take each legend to the place where they first met and had sex. For Mewtwo it would be in the dark cave where she was in darkness and despair. She would remember this place as only having one positive memory and that is meeting the MC and being shown love for the first time. It is only when you are in complete darkness can you see the light the clearest no matter how small it is.
Another example is for Celebi in the forest BUT she does not want to just remember when they just met but see it as well. She takes the MC to that spot and travels back in time with him to that moment and hides to watch. She gets in the mood and they have sex at the exact same time with there past selves down to there moans and thrusts to orgasms.
Had thought of something Mewtwo could say to MC but don't know where. She says "I will always love you and stay by your side even if you cant change your dick. That is the cream on top of the cake though I prefer it in the cake and I do feel a little empty right now." Sex ensues and she says "I never get tired of this and never want to".