Played through the current build, and have a few ideas. You may have already considered them, but I thought I might put them in just in case. In no particular order...
1. Spirits propositioning relationship upgrades. Similar to how Ori will sometimes ask for sex in Chat and Ori and Flo have chances to initiate during cuddle/make out, past certain thresholds (most likely differing between characters) Ori, Flo, and Sean may on their own initiative confess their feelings which the player is free to accept or reject (at a significant relationship dip due to heartbrake, so the players themselves can't just confess immediately after like nothing happened). My opinion is that it'd be best to occur as a morning camp visit at random chance similar to the Ori/Flo event, with heart thresholds at least slightly higher than it takes for the player to confess to the spirit, and they won't propose again if rejected so that the player can still forge a significant platonic relationship.
2. Harem dynamic, meaning if the player is boyfriends/lovers with multiple spirits those spirits will acknowledge it. It could even be that the player will have to talk things out between them (or catch them talking things out with each other in relation to Point 1) before a confession will pan out. And while it would most likely be too much to make properly interactive you could make some narration for multiple-partner events, the most obvious one being being lovers with Flo and Ori and engaging with their recurring event.
3. Inviting spirits into the tent, and sex in the tent. The later is obvious for what it would be, and I can see the former being available when a spirit visits camp at night. This could lead to platonic snuggling at friendship levels, and aforementioned sex either to bed or as a morning surprise at boyfriend levels. Ideally you should be able to invite spirits earlier and earlier in the day with the increasing severity of the weather (and intimacy of relationship), and in conjunction with Point 2 able to invite multiple spirits for group cuddling/fucking.
4. Occasional descriptions of events involving background spirits. These wouldn't actual effect anything, but would show how the denizens of the forest warm up to the player. Tied to the relationship values (though not relationship status in conjunction with Point 1) and the spirit tree, spirits in descriptions will be more visible and less wary as the player's reputation improves. Notable events may include curious visitors to the player's camp (initiated during search when the player would otherwise find nothing), accompanying the player briefly (initiated during travel), and spirits offering warm reception when the player visits the lake.