1. Everyone wants more hooks OR free placement of items. It might make more sense to have more (and less restrictive) hooks inside the house; being able to put as many trees as one desires in the yard, and to have free placement, would be uber cool. (Note: you have free placement of barricades and catapults in Helm's Deep, so it can't be that hard to put a tree where you'd like it in your yard.)
2. Have functional chairs so the owner can sit at a table, and in fact have multiple chairs so friends or kin can visit and sit. For kin houses it could be cool to have a lot of chairs or benches where people could sit for meetings or musical concerts or whatever.
3. Allow a stable to be purchased and then attached to the outside of the house, and permit one or two horses (of a type currently owned by the player) to reside in the stables. Maybe a one-horse stable for standard houses, two horses for deluxe houses and some higher number at kin houses.
4. Somebody mentioned pets (like a dog or a cat) which could be interesting. Taking that idea further, what about an NPC spouse inside the house or sweeping the front porch (both genders available) and maybe a couple of kids playing in the yard during daylight hours?
5. Put more NPC's in the neighborhoods to make them look more lived in. Perhaps there could be a formula that increases the number of NPC's according to how many houses in the neighborhood are owned and occupied.
6. Finally, this is probably a pipe dream but I’m going to say it anyway. In Disney’s Pirates of the Caribbean (my kids played it) you can play cards and gamble as a completely separate pastime to killing things. This could be a marvelous addition to kin houses. Hey, if Disney can do it surely Turbine has the chops, right? Card games are about the easiest things to program (since the 80’s!) All it would take to "enter" a game would be an empty chair at the card table. Choice of one or two games, impossible to cheat, betting limits established by agreement of all participants before a round begins. Kinships are social organizations, and kin houses should have a greater purpose than trophy displays and storage. Plus, between killing orcs, card gambling with friends would be fun.