In other words, give us something else to do with other people in our homes to encourage group-visits. I'm tempted to call the idea "gamemodes" a-la GMod, because to be perfectly earnest, that's 100% from whence I stole the idea.

Suggested "Party Games"
1. Tag, Freeze-Tag, and Hide'n'Seek -- pretty self-explanatory in concept, but there would/should be some constraints. For example, a 30-second countdown for whoever is "it" that both blinds and paralyzes them at the beginning of each round, giving hiders and "not-it" players to gain some distance. There are also ways ESO's mechanics could allow us to play these classic school-yard games in very interesting ways. For example, I'd suggest allowing all flavors of stealth, detection, and evasion precisely as they are in PvP. I'd also strongly suggest a "light-melee-attack-only" policy to tag a player (with style points for only using your fists). And Freeze-tag could easily employ the CC-lock mechanic.
2. Enchanted Furniture -- (like GMod Prop Hunt) it would be extremely similar to Tag/Hide'n'Seek in its implementation, but players can hide by choosing to become any piece of furniture they find that was already placed within the house. One player is "it" and tries to find all the other players masquerading as items throughout the house. The person seeking only wins if he/she found everyone who was there to find.
3. Magical Clone Shuffle -- (like GMod Guess Who?) this game would populate the map with NPCs created from all the player's character models. The NPCs would wander the map according to static, but randomly selected paths they follow. There should also be a small chance of randomized behaviors, path switching, and emotes from NPCs to keep the person who's "it" guessing at who's-who.
4. Who's the Assassin? -- (like GMod Murder) this game would randomly assign all players secret roles. There would be one fighter's/mage's guild member who is armed with either a bow or a destruction staff that can instantly kill any other player. There would obviously be a secret assassin armed with the Blade of Woe. Everyone else is an innocent bystander who is unarmed and cannot attack/kill anyone else. The assassin loses the round if that player is killed by the player assigned to be the Fighter's/Mage's guild member. Perhaps the guild member player should lose/die via ethereal smiting for guessing wrong and killing bystanders too many times. Obviously, the assassin wins by successfully murdering everyone.

Anyways, there's my suggestion: party games. Timed rounds and an instanced scoreboards are also good ideas for these types of games. To put it simply, without more stuff to do with friends, there's no reason to group up and go visit a house except for guild events, small dueling tournaments, using a practice dummy, maybe some crafting, or just showing off your mad interior design skills to your pals. Party games would be an awesome additional feature.

Consider how such party games would be an incentive for players to get a large or manor home. Consider how much variety new features like this would add to ESO's game-play. By no means do I think these things would be easy to accomplish. I'm simply suggesting these features for consideration sometime after Morrowind launches.