A graphical web-based game where you're invited to stay at a family's mansion for a week. You can befriend them or be a jerk and make enemies. At the endof the “week”, you get a different ending depending on how you acted. Basically, a Choose Your Own Adventure type game, similar to all those Japanese fantasy games that I honestly didn't know much about when I came up with this.
The game plays similarly to Shadowgate, Deja Vu, and Uninvited (NES games built on one engine) but the NPCs don't stay in one place.
This could go in any direction – Mystery/Drama, Magical Fantasy, Hentai (“This is my daughter, who just turned [insert legal age in your country]!”) Prefer to keep it PG-rated, though.
The game is falsetime-based. There's a “clock” and certain actions eat up “minutes”. The characters move to different rooms and do different things at different times, making it impossible to meet every possible interaction with every character.
After a time, your character gets tired and has to sleep, gets hungry and has to eat, needs to head for the restroom, etc. This affects your ability to interact with the NPCs, since going to bed late makes you wake up late, the family thinks less of you if you miss dinner, having an interaction interrupted by running to the nearest restroom can make the NPC think poorly of you and cost you time needed to finish the interaction.
In my visions of this game, a Wife never came up; there was the Father, the Young Son, the Daughter, and the Maid/Cook, but no other characters. So let's say the Wife died for some odd reason, it adds a sympathy factor to the game and gives you a storyline for the Father.
Taking certain actions will decide your character's attributes. If you consistently choose to be rude, you may not be presented with the option to be polite in the future, as an example.
The daughter is practicing her tennis skills at the mansion's tennis court. You talk and get around to the offer of a game. You can make different choices based on how good a tennis player you decide to be and whether you decide to go easy or hard on her (if you're good). Different choices have different outcomes and will affect future options, especially on the tennis court.
Savegames could be implemented with cookies, with a Sekrit Code, by u/p combos, or some other method. I prefer a code in the URL, don't care how easy it is to crack. http://hamburgsteak.sandwich.net/house.cgi?status=11aXg234FDsagd or some such where the code sets memory blocks is good enough.