First, for any house within a city with an outside area that looks out on said city, like Highhallow Hold and its balcony or Water's Edge, add NPCs walking around the streets of the outside-of-map areas, plus the sound effects of a city that's actually alive. When you're on your balcony looking out on the city you live in you don't expect it to be eerily silent and devoid of all life right? They shouldn't be actual NPCs that take up precious furnishing slots though, just animated background art and location-appropriate sound effects.
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.