I really like this game and have some suggestions that you can feel free to use.
1) Being able to buy a house that comes with a maid (or a guard/companion like Lydia in Skyrim) that you can do lewd stuff with and that has similar mechanics as the shopkeeper. This would motivate people to save their gold to make a big purchase. You could also decorate the house with items you buy or earn through quests/romances. You could also store items in a chest at the house.
2) Having an NPC that you can convince to take off her clothes, or possibly for the most recent character added. With the newest character added you could make it so that you need to advance her quests and she will be fine being an exhibitionist and doing lewd things publicly (corruption-type mechanics).
3) An NPC where clothes aren’t stealable but you fight them and break them off. Or where you buy the clothes off her (like it’s a glitch in the game where you buy things from her and you can buy her clothes or something like that).
4) A beach area or hot springs/bath. You can have an NPC here that you rub suntan lotion on or wash with your hands and have a minigame similar to orc massage. Or you bring the most recently added character to the beach or hot springs/bath and do the minigame with her.
5) Companions for quests or that you can bring with you around the map. They could react if you do lewd things with other characters.
6) A big-breasted character, or multiple new characters with big breasts, just to change things up with the characters.
7) Possible corruption mechanics (I mentioned it a bit earlier). Eventually being ok with doing lewd stuff publicly, first being corrupted into wearing only underwear and then being nude.
8) An outfit/clothing system where you can change the clothes for characters (I think you mentioned you’re already planning on doing this).
9) A queen/princess character and having characters that can interact with each other. Such as having the royal guard and queen interact.