It's a sickly horse that I've beat a couple times over the years, but one I can't not bring up, because I really do love this concept:
So, there's a series of extremely obscure japanese RPGs called the "Agarest" series... and they're really bad... but they have this one awesome gimmick that I've never seen anywhere else.
Basically, depending on the game, its story takes place over the course of 3-5 generations. the protagonist of each generation has a point toward the end of his individual story where he chooses a wife, and the wife chosen dictates the appearance and fighting style of the hero of the next generation. Pair up with the squishy mage chick? Next generation you have a magic knight. The no-nonsense warrior lady? Next generation you have a heavy-armor tank.
likewise, there's a niggle in the story that at the end of all the generations, if the hero isn't born, he won't be able to fight the final villain, and thus the world will be destroyed. What this means is that the girl a generation's protagonist wants to procreate with will be willing to do so, even if she's not super-duper fond of the guy, which is where the dating sim elements (because, again, niche japanese game) come into play. The more the parents love each other come the end of one generation, the higher the stats of the offspring next generation. I... don't know how that works, really, since both parents die after the baby is born, but it adds more customization to the game regardless.
between those two systems, the game has a mechanic that creates a ton of replayability just to experience the different character builds and relationship options, as well as a system you can use to create a handicap for yourself if you're the kind of gamer who's into that.
...or rather, the game has a mechanic that WOULD create a ton of replayability... if I could conceive of a scenario where anyone would want to subject themselves to a second playthrough of an agarest game.
