I don't know if it would be fantasy or that over time and in the games I've seen, it has caught my attention.
1. That you, out of nowhere, appear somewhere beaten and with no memory, suddenly some schoolgirls find you and want to wake you up, you wake up and see the schoolgirls and their panties. Then you pass out and one of them takes you to her house, a rich people's house, she lives alone with her younger sister and her single mother. They “adopt” you while you recover your memory and work in their house, with time you recover your memory, and you also corrupt each one of them, eventually they fall in love with you and you have your secret harem.
2. That a young girl comes to your house lost at night, a house that is outside the city, a house that is in the countryside, and that the lost girl asks for your help to give her food, bed and a place to spend the night. Eventually, the two get to know each other and become “curious” about each other.
3. You are new in a small city with few inhabitants, you live in a place where the houses are very far apart from each other. The second day you arrive to live in that place, some people knock on your door, and they turn out to be a mother and her youngest daughter, the mother, with a husband absent on work trips to other countries, feels attracted to you, and a daughter who, the fatal father figure, has taken an interest in you, who always comes to look for you, just to play video games.
In short, those are the fantasies I have seen in some games and “manga” that caught my attention; however, many were never finished or started.