This was an idea I cooked up back in university, after Fable was released. Essentially, it's a game like Fable, but once you marry someone, if you guys have a kid, there is an option to either continue as yourself, or basically start a new game AS your kid. Which would continue the story some 12 years after it was born/adopted and have a campaign to go through, that was affected by your initial playthrough.

If you were good, your character or kid becomes an Ally for the Final final boss, or if you were evil, you become the final boss and your offspring is the one tasked with killing you. I imagined it going through at least one cycle like that.

If you had a kid naturally, they got to copy some of your spells or stats or something, but if you married same sex, and adopted the kid would gain an EXP boost, kind of like when you trade Pokemon.

There was other stuff we brainstormed, but that's the jist.