sgtpopper

Different genre, and it's not interactive but: Enslaved had some very nice scenes when it came to the ambivalent relationship between Monkey and Trip (well, at least in one instance that I can remember). So does The Walking Dead, and that one is 75% about dialogue interactions. So those could be good sources of inspiration. About the "buying love with gifts"-thing: If it is a game system (i.e. works for everyone and their gift preferences) it degrades characters. On the other hand, it makes more for an open-world feel and allows the player to experience his/her very own, unique story (the world vs. story thing, player-story vs. designer-story, some people prefer one or the other). Other people prefer to experience a well-written narrative and don't mind having their options reduced...it's a matter of taste and priorities, both have their appeal and downsides. Personally, I think making somebody a gift could be nice if it's tied into the plot or is otherwise made significant (e.g. requiring to to learn or make use of some crafting skill and finding the proper ingredients. The PC makes an effort that is not about getting better gear/xp, but rather the reward is relationship options, a little dialogue, something along those lines). That's when a present makes sense, when it's not just a resource that is farmed/stumbled upon, but part of a plot decision (e.g. some unique item and you can choose to give it to companion A/B/C at one point in the game).

I agree, don't make the main quest about saving the world...