Picture of Akane having sex with her father
There don't seem to be very many good stories involving incest in the Ranma 1/2 universe, with perhaps the exception of Genma and Ranma. Unfortunately, a Genma/Ranma pairing doesn't do much for me — probably because a pairing of Genma with anyone doesn't do much for me, unless I'm in a certain mood.
Mind you, there are such stories out there. But it seems that they usually don't manage to bring me into the story, even to the mild degree needed for porn. Somehow they don't manage to sell me on their set up.
What I'd really like to see is a story in which Soun has incestuous desires concerning his daughters, but in a subtle rather than an overt fashion. I.e., he doesn't have out-and-out sex with them, or engage in obvious peeping. But he has insisted that daughters require discipline so long as they live under their father's roof (and maybe beyond), and that proper discipline is administered upon bare bottoms (or perhaps even other bare areas). He insists that proper Japanese families bathe together. He monitors their hygiene by watching carefully while they bathe, and bathing anything himself where he feels they are too lax. He monitors their health by such practices as regular measurements, at least of breast size, and possibly also other aspects such as areola diameter and pubic hair extent. He insists that pubic hair be trimmed, or even entirely shaved, and the girls never do so sufficiently well themselves, so he must always touch it up. There is a spectrum of possibilities on which he can fall here, from only slightly abnormal behavior to utterly psychotic.
Then Genma and Ranma show up. Genma is merely confused by his old friend's behavior, doesn't know what to make of it, but advises his son that it is a father's right to raise his children as he sees fit. On the other hand, since Ranma must marry one of the girls, Soun starts to draw Ranma into his eccentricities. Ranma slowly metamorphoses from being weirded out to becoming addicted to these games.
I don't really see Soun playing his games with female-Ranma. Ranma won't have had the years of training to submit to such behavior that the Tendo girls had. Soun might try a little bit, like measuring Ranma's breasts, but I think Ranma would put a stop to it. And in any case, Soun is concentrating on Ranma being his male heir. On the other hand, I think he would take advantage of Ranma's female form where his daughters are concerned. For instance, Soun could insist that Ranma accompany his daughters (in female form) in public bathhouses and the like, to enforce his standards in locations where he himself cannot.