I've had an idea in mind for a lewd game for a while. It's primarily an incest fetish game, but there's room to work in a lot of other fetishes and themes. The setting is on an interstellar colony ship making a centuries long sub-light voyage to a new world. A large number of the colonists are in hyper-sleep, though some trained specialists are crew who are scheduled to be awake for a 20-30 year career before retiring back to stasis.
The plot starts with the male MC being woken up by the survivors after a collision. Life support and control are a mess, but the fail-safes on the sleep pods and the drive worked, so the mission is still possible. They woke the MC because he's the only person with officer credentials the surviving crew could manage to wake. This makes the MC the new acting captain with override control of the remaining controllable systems. The twist is that the access to wake up any additional crew is currently lost, and the few other surviving crew are women. The only way to keep the ship crewed long term is to turn it into a generation ship and make the next generation of crew the old fashioned way, at least till certain systems can be found and repaired. This isn't as genetically dangerous as it would be in a normal population, as the colonists were screened for all known genetic defects during selection. The REAL twist is that the MC will turn out to have an undiscovered genetic defect that he can only produce X chromosome sperm. No new males can be born. As the only accessible officer, and without the current ability to appoint new ones, the MC will need to spend most days back in hyper-sleep. For him and thus for the player, 18 years could pass by in a few weeks or months.
It would be part resource and personnel management sim, with strategic decisions on what projects to pursue, who to train on what, making sure there is enough food, water, air, quarters and etc for the crew, keeping up moral and so on. It will also be a dating sim game. Just because it's maybe necessary to reproduce with the MC doesn't automatically make it palatable. For the survival of the mission or not, you don't have the authority to order someone to have sex. Your best strategy would be to maintain a good relationship with each crew-member, taking into account their personality and working to build their attraction, affection, loyalty, and/or (perhaps eventually) devotion to you. If things get too bad, or you push things too far too fast, they'll ignore your orders and while you sleep, work to find a way to replace you after all (various bad end scenarios). Each character will have different personality traits, aptitudes, etc, some of which will be inherited and some learned during upbringing, both from their mother and teachers while the player is asleep, and by interactions with the MC when awake.
Over time you'll be able to push the culture in new directions. Crew who support you will be more willing to go along with changes, though their own traits and norms will affect what they can accept. At the start the age of consent is 18, incest is icky but maybe necessary, nudity and sex are private, and nobody is into any kinks (everyone passed rigorous physiological screening to join the mission, but that can change). Ethics are maintained, work uniforms are worn on duty, and orders deemed unreasonable would lead to mutiny. Over time though, who knows how far things could shift? Will you make an efficient and equitable bi-sexual harem utopia, or perhaps risk the mission to do something twisted, like turn your decedents into a nudist loli slave cult or something?
Art style for the characters would be a large variety of drawn pieces resized, mixed and matched paper-doll style to show the various character's physical characteristics and current clothing. You'd design your own character at the start. For balance and fun reasons, the MC's children will always be more likely to inherit genetic traits from the mother, There's more I've thought about, but that gets the point across and I should already be asleep.