So this is a pipe dream that's going to be impossible right now without way too much work, so consider this "what I want added for COC3" rather.
I really wish there was some way for our sexual skill to grow, and be reflected in scenes. Let's say sucking dick, if you come as a default non-experienced (meaning non-courtesan, who'd start all the sex stats at a medium level) and try to suck a dick like Arona's let alone Kas or Brint, you start gagging and coughing and can just get the head and a little bit more in. Add half a game's worth of cocksucking experience (or having done Evelyn's content I guess...) and you're deepthroating Kas with no gag reflex like you got an itch in your stomach lining you need scratched. Dickmanship, same deal, first time Cait takes you between her plush thighs you just rut away with no real knowledge of what you are doing, like a teenage boy after prom, complete with a premature ejaculation and probably some soothing totally not mocking words from Cait. But after proper education (read: dicking down every hole and anything that vaguely looks like a hole halfway across Savarrah) and you know just the exact places to thrust and send that kitty to the heavens every time and leave her a blubbering mess hopelessly addicted to your nethers. As said, you come along with Courtesan background (and possibly a "Sexually Skilled" sex perk) all those levels start at basically "Mallachite" levels.
Could possibly add effects to that, like if you just beat up Byvernia the other day and left her a gibbering mess afterwards that's done little but dream of you since (meaning, you picked a sex option where you had a high skill), she gets a status effect applied to her next when you meet that puts a debuff on her Focus defense for that fight, because she kinda doesn't mind losing again. If you don't dick her down then, or otherwise leave her unsatisfied, she doesn't have that debuff next time (or even better, if you leave her humiliated and frustrated she may show up with a buff next time, wanting to pay you back properly for that, removed if you were to either beat her down and satisfy her, or she beats you up and claims what's hers). Could even do that as a stacking debuff, where if you get to 3 debuffs, next time you run into her she just asks you if you want to skip the fight and just have sex instead, which in turn on some unique characters could open up a friendship route if you do just that (or if you don't, she walks away in a huff, then comes back with that buff next time you meet her).