Skyrim and Fallout 4, for sure. But I would also change the story for both of them so that the main character weren't the most important person in the world. Slap in some bath houses like in Kingdom Come Deliverance, and especially for Fallout 4 add the ability in base building to make bases into harems, bath houses or brothels depending on what relationship you want to have with your settlers. Anyway, i would make speech/charisma more important as it lets you seduce people to join you.
I would also love to make a lewd Persona 4 or 5 game. You telling me that during the most hormonal and horny time of our lives, no one in these little polycules would try to smash?
Baldur's Gate 3 was already pretty well done in the lewd department, but I want even more. My maxed out charisma Bardlock would try to breed anything with a wet hole if I could design it my way. The epilogue would be epic - so many babies to help the world repopulate after mother brain.
Stardew Valley would also be on my 'to lewd' list. Though I'd try to change it to a 3d game instead of 2d pixel, and there would be a lot of 'animal husbandry' potential.
Become the village slut and be the rumored father of half the kids in the valley, or the home wrecker that cucks every wife in town with their husbands - or be a predator lesbian that turns all housewives gay. Get married and introduce your spouse to the joys of 'playing' with horses, dogs, cattle and whatever else you have on the farm. Grown psychedelics and host orgies with the towns folk that like you. Use the witch's magic to make the whole town forget all about your depravity and start the cycle all over again.
Was there ever a good porn version of Tomb Raider?
Beast in the Sun is the closest we got. But it's very slow in development.