A lot of IPs would make for good porn games. Star Wars, Star Trek, and Farscape as two you mentioned. Stargate would be good too. A lot of IPs already have porn parodies out there too.
If I had to pick one that I got to fully control everything about it... I'd go with Star Wars. You'd be a jedi on the run, it'd be set between Episode 3 and 4. You'd build a crew (all sexy ladies, with maybe the exception of the token droid companion) and try to keep one step ahead of the Empire. An Inquisitor (sexy lady you get to win over eventually) is hunting the crew but so are the ISB, the Hutts, Black Sun, and other antagonists. I have lots of ideas for missions and plot points, but ultimately there is no grand galaxy saving plot, no big confrontation with the Emperor or Vader, just a crew trying to survive. It would be largely open ended. Kind of like the old MMO Galaxies Edge, but with a little more overarching plot.
As for gameplay, if I can go all out here, it would be a semi-open galaxy with large open worlds to explore. You'd have freedom to go to dozens of worlds at any time. But your action would matter. If you kill a bunch of troopers on one world, then you'd be more wanted, and it'd be harder to go unnoticed when visiting another. The main goal is to do enough to get by, to keep a balance between the light and dark inside yourself and not get killed by the Empire or other hostile power. There would be character creation, you could play a male or female. Though I wouldn't have any male love interest, so I hope you're into a lesbian harem if you make a chick. And it would be a harem game. You could play a few different races, but they'd mostly only be the near humans. Zabrak, Mirialan, Umbaran, Twi'lek, Zeltron, Pantoran, maybe the Chiss but probably not. One or two others that are human enough to not be weird. You'd be able to customize your ship and lightsaber. As far as fetishes go, harem of course and you'd have all the classic vanilla stuff. Plus, incest, pregnancy, cheating (you'd get to steal a girl from an NPC guy if you want), some light sub or dom (your choice), alien girls (duh), optional lesbian stuff. Nothing extreme, no rape or NTR or scat. And the game would come first, the porn is just there to add to the flavor.