There are a lot of mechanics that can be added to such a game, if we talk about gameplay ones:
-Renown: Give the MC a feeling that the better he is as an adventurer, the more known he is and get some priviledges because of it like discounts and ability to buy premium items.
-Getting a house and improving it: We tend to have a lot of money because of the quests so a good way to spend it would be to improve your house and get some permanent positive effects because of it so that the investment is meaningful.
-Buying slaves and getting servants for the house: When the house starts to feel like a mansion it's gonna need to be maintained.
-Receiving visits to your house and social interactions: Most games just expect the player to do all interactions, but it would be nice that people could actually visit and interact with you, or that when you enter the adventurer's guild you may be interacted by others.
If we talk about sexy mechanics it depends on what type of game you are making, will generate some context for the mechanics:
-Corruption game: Let's say that for example the game has as the central conflict that 3 futanari demons are trying to make the town into a place of debauchery or that they simply want to corrupt that place for whatever reason, a good mechanic would be that you can feel the danger of their actions, for example:
Let's say that the town has a church and the priestess needs to travel once per month to report to the main church and come back, one urgent/hidden mission that can happen is that the priestess gets their convoy attacked and if it isn't saved one of the demons may take the form of the priestess, come back to the town and little by little corrupt the church into a cult.
Having to deal with the demons strategies and having some effects makes them see more of a challenge and problem, we lack this element in most games.
-Chill game: Overall in this setting I would just hope that the girls act based on their personality, if a character is dominant, he/she should act in that way, also it's fairly rare that there can happen NPC x NPC romances or stuff, there are very few games that try to make NPC's a bit wholesome, even in games like Coc 2, tits and lilith throne while some NPC's can have sex with each other it is never seen that they may actually become boyfriend/girlfriend or whatever, basically that they enter some relationship.
-Harem game: If the game is going in the line of gotta catch them all at the very least it would be appreciated to actually have to convince girls to join the harem and having to deal with harem issues and conflicts.
-Manipulation game: Basically I mean a type of games in which NPC's can actually try to do some bad stuff to the MC or other characters, be it transform, mind control or basically influence them in any way. In this types of games it's better to not easily defeat said NPC's but give them time to shine and plot, having some tactics that they can keep trying until you manage to totally defeat them.
-Sub/dom game: Basically a game that takes seriously those rols and will have characters try to impose their will in that way, in that type of game it would be fun to be able to tame dominant characters or let an otherwise submissive character awaken her dominant tendencies.