First, for any house within a city with an area that looks out on that city, like Highhallow Hold or Water's Edge, add NPCs walking around the streets in the out-of-bounds areas, and add sound effects of a city so it doesn't feel like a ghost town. It's for immersion. They don't necessarily need to be NPCs with paths, just background moving assets. I'd love to stand on my Highhallow Hold or Water's Edge balcony and see and hear people and mounts walking around accompanied by the sound effects we hear when we're in the city itself.
Second, how about our other characters as houseguests? We need an increase in the number of special furnishings we can add for sure but I'd like to be able to see a random other character of mine walking around my house.