I'm not sure if this is exactly a fetish, but I've never seen a game (porn game or otherwise) about resolving the genetic issues resulting from incest that would fundamentally change a society, in which that process happens during rather than before the beginning of the game's story.
What I mean is: the game starts, either in the present or the future (or I guess maybe the past but in a sci-fi-ish alternate timeline's past), when incest is gross and icky and makes gross inbred babies. But as the game progresses, either your main character works to resolve that problem or broader society does so. As a result, people suddenly are dealing with the possibility of not just it becoming non-hazardous to nail a blood relative, but even legally and potentially socially acceptable during the course of their lifetime, and the gradual acceptance of people marrying and having viable normal children with their own moms, dads, sisters, brothers, grandparents, kids, etc.
I think a narrative like that would be very sexy, especially if it wasn't just doing inbreeding in a lab but a person's actual DNA being cleaned up to the point where they could just marry and breed with any relative like normal but without the risks of inbreeding to their potential offspring. Then, there's narrative tension as not only society and laws struggle to handle this new (essentially acceptable) way of life, but also in the relationship itself as the people involved deal with the fact that love and compatibility and material needs really are all that matters in such a world, and what family becomes when your own son might take your own daughter to prom (for example) with the real intention of fucking her, and having that be okay.
