I have a small feature I'd like to see: It's nice to see when spirits are the ones to prompt getting intimate. So far, that's only Ori and Haru during makeouts, and talking to spirits informed of your birthday during your birthday.
As for what I'd add... Mostly just letting Flo and Sean do it during the makeouts, to start with.
You might add a smaller chance for a spirit to initiate during other kinds of romance, or during chats- If the make-outs seem to be a 50% chance to roll a prompt, maybe closer to 20% for other romantic gestures, or 10-5% for Chat if you're already intimate lovers. (You could make it more likely the longer it's been since you've had sex, but that would require a new timer counter. I'd rather see a reuse of the existing system for a smaller update, for something more plausible.)
And on the unlikely side of things, during the background-spirit minigame, you could have a chance that they'll want to mate with you right away, if you've already succesfully wooed a lot of background spirits. (Word gets around, I guess.)