I'm not sure if this would qualify as "small" or not, but I would love it if there were some proper "in law" relationships, that actually affect how sims interact. I play multi-generation a lot, and one thing that constantly irks me is the lack of multi-generation in law relations.
This isn't just hoping for cutesy interactions or anything, it's actually vital, because of how flippant romance is in the game. For instance, I was playing with a jealous sim recently--a kind of difficult trait to play, but I don't mind the drama too much. What I DO mind is how he gets "jealous pangs" every time his wife talks to his GRANDMOTHER. And many times I've had sims get flirty or have whims to do romance with their in laws, since they tend to have friendly relationships with them, Which I wouldn't mind, if the game had them treat this as an actual thing, like some kind of "Omg! I can't believe I'm attracted to my brother-in-law! Ack!" moodlet. Ideally, I'd like romance to still be possible between in laws, but much less likely for autonomous actions, more difficult to succeed at romance actions between them, and some kind of beefed up embarrassment moodlet related to it.
Anyway, this might be a more complex overhaul, but basically any sort of improvement to multi-generation and extended family relations would be my top priority.
