A true sandbox game. Start with a point and click adventure game interface where the characters are 3D models. You can create your own character. He can be a young college creeper or a scruffy dirty old man. There would be soft RPG mechanics related to the job sub-systems. It's set in a small neighborhood and as you go deeper into the game you unlock more areas. Each of the girls has her own distinct personality, likes, dislikes and kinks. Probably a ton of behind the scenes stats. You would effect these stats either trough conversations, giving her gifts, inviting her out places, or playing with her.
The dialog system would be similar to a visual novel where you can make choices through the conversation and these effect the ultimate result of the conversation (base relationship stats manipulated, maybe some background stats about preferences slightly effected). And the dialog system would have a ton of generic dialog but with a bunch of small flourishes added procedurally to the text to give it a hint of the girl's character. Each girl would have a bunch of her own specific dialog scenes too,
As you progress your relationship with them you can purchase them clothing and other gifts. Most of the clothing items at the start are fairly normal stuff and the girl will wear the clothes in her inventory based on her preferences, but the things you do can affect their preferences slightly (and as the girl gets more corrupted she will be more willing to wear sexier and sexier clothes). The reason it's important for the characters to be rendered as realtime 3D models is so they can have different clothes swapped in easier.
When you interact with a girl you get a menu with like "talk," "ask," "gift (if you have an item)," "kiss," "fool around with," etcetera.
There would be options to either play adult scenes out as cut scenes where maybe you get a few dialog options about "doggystyle or reverse cowgirl?" or "cum inside or pull out?" Or you could play them out as a sex minigame. When you kiss a girl it would display her shoulders and head and you can click where you like, and different girls would react differently to being kissed different places (and this would also depend on their relationship to you). There would also be a menu on the side to just play it out as a cut scene and have a generic response. Next you would unlock a groping scene. Being 3D would help with this in a few ways too. You could drag say a boob around and it would deform in the directions you're pulling it, or you could click on the buttons of a girl's outfit to unbutton it (maybe she freaks out, maybe she lets you). Ultimately it would all be in service of a sex minigame in the style of the old flash games of decades past. Tease the girl to get her horny, slowly undress her and have your way with her.
In general the NPCs would be guided with something akin to Oblivion's radiant AI system where they have hidden desires. They have a rough schedule of where they want to be and when and this is altered by hidden stats like hunger and fatigue and like desire to pick flowers or whatever.
There would be tons of little sub-systems I guess modeled after like Stardew Valley. There's just like little odd jobs you can do to make money. For some of them you can take a girl along if your relationship is good enough with her and you split the money. Maybe a girl likes to go fishing so you can take her out on a boat and this will increase both your relationship with her as well as get you some money. Maybe a girl wants to be a model so you get a nice camera and take her to a nice location to take pictures and then they're rated and maybe you make some money off them. And then once she's comfortable with you taking photos of her...
