Ideas inspired by playing through the current build:
1) Visiting Ori's house, or at least a background art when relevant. Possibly art for the houses of other characters as well.
2) Introvert Mode: An option for the spirits to interact with the player on their own. This means that in addition to visiting the player's home, they may interact with them while in the same area, and may engage in the various interact options and even ask their own questions to let the player flesh out their own backstory. They can also end conversations like they do when at home/night, portraying how the player cannot simply monopolize their time. The final feature would be influenced by affection, though should not be possible to eliminate outright. For the random spirit encounters, they will offer something random weighted towards the highest tier. The purpose of all of this is for a player who wants the relationships to develop naturally according to their own pace, rather than just going Gift×2+Play×2+Romance×2 every day.
3) The option to save the customization of random spirit encounters. For the times when you find a spirit that looks good, but don't want to go through recreating every feature.
4) If I remember right you already said you wouldn't, but in case you're willing to reconsider after much time has passed: Survival Mode. While not outright lethal, considering you can eat food, having a reason to would be good. Failure to do so will cost time as a spirit ensures you have a hearty meal, same as not getting enough sleep. Speaking off, even without collapsing you can still be fine with only 5 Hours every day. A build up of fatigue as your overall energy diminishes would be inevitable. And finally, running around on rainy and stormy days is just begging for illness, which could also be an opportunity to be taken care of for a day.
5) Spirit Stone lets you find unique things in the Overworld. Essential just, it's revealed by your cursor, you click, you get. Could even be quest related.
6) Event: Spirit For A Day. Find a magic item which gives you the appearance (and only appearance) of a spirit only for a day, and allow the player to use that to have unique interactions with the characters both in seeing what they think of the new you and/or pretending to be any old spirit to see what they say about you behind your back.
7) Random chance to find clothes in Search. If Sean can do it then let the player get lucky as well, as they already can when fishing. If it is already possible then I'm surprised at myself for not having had that experience.
8) Multiple-character interactions. For those times when characters are in the same area as each other. These would not be anything special and would be repeatable, but would show more of the relationship between characters.
9) There is a chance that if there is a character in the area when the player is engaging with a generic spirit, if they have sex in such an encounter the player may be confronted for being a horndog. The nature of this confrontation would be different for each character and level of relationship, and they player will have different options to influence the interaction positively or negatively. Perhaps this could be related to a general acknowledgement of the player's sexual interest. After all, a Flo that knows you wouldn't mind getting into her nonexistent pants would perceive you differently from a Flo as innocent as flowers, unless she suspects that the entire time in which case please put that information in the game because that would be some significant to understanding the character.