The Sims Medieval unlocks new furniture (and clothes) as you gain achievement levels.
Civilization, where the player's achievement of milestones would entitle the player to add additions to a picture of a palace. Civilization II did the same shtick with one's throne room.
Assassin's Creed II: The player's house obtains portraits of assassination victims, and the game encourages the player to buy art objects for his house and other improvements to the town in which the house is located.
Follows over into most follow up games but gets less focus as time goes on.
It started in Gold and Silver, where you could decorate your bedroom at your mother's house.
X-Wing Alliance fills your quarters, which are absurdly spacious by any military standard, let alone that of a rag-tag rebel fleet, with souvenirs from all your family missions—your first TIE kill, paired swords (a gift from your uncle for saving him), a piece of debris from a Black Sun fighter with the logo intact. Also, your flight suit helmet has different decorations depending on the number of kills you have.
In Legend Of Queen Opala, you can buy and upgrade a house. If you fill the art gallery (by stealing paintings you can find all over the world), it unlocks a bonus island.
Fable I and Fable II. In the first game, certain bosses you defeat earn you trophies. They can be placed in any house you own (thereby increasing its worth), or you can show it off to others for Renown.
Age of Empires III has the decorations for your Home City. Too bad these are only for European powers, and even then they are not very pronounced.
Alpha Protocol has several items and trophies of Mike's foes pop up in his safe houses.
Fallout 3 has the ability to purchase "themes" for both your Megaton House and your Tenpenny Tower suite (depending on whether or not you killed dozens of innocent people for Tenpenny's amusement). These aren't granted automatically, but as they are almost purely cosmetic, most players wait until they have enough bottle caps to bathe in before purchasing them.
The Bobbleheads you obtain can also be put on display.
Similar to the preceding entry in the series, the Lucky 38 presidential suite from Fallout: New Vegas has a display area for any snowglobes you acquire. You can also purchase various additions for the suite, which do serve some practical purpose by giving you more storage containers and a workbench for crafting.
In Professor Layton and the Curious Village, as you progress through the game, you acquire furniture and other decor items, with which you must decorate Layton and Luke's rooms at the St. Mystere Inn. Each item is best suited to either one or the other; completing both rooms unlocks extra puzzles.
In Professor Layton's London Life, a bonus sprite RPG packaged in with some regions' version of Professor Layton and the Last Specter, fulfilling some of the quests assigned to you by other characters in Little London will earn you furnishings, draperies, wallpaper, and other accessories for your little domicile.
In Professor Layton and the Miracle Mask, completing downloadable puzzles will allow you to pick furniture items for a room. Occasionally, characters from the game will appear in the room as well. Once you've filled up the whole room, completing further puzzles will give you second chances to swap out the items you don't like. And when you've solved all the downloadable puzzles of a given type, you earn a bonus item.
Saints Row 2 has crib upgrades. They actually do something: they increase your reputation so you get more missions, at least until you're swimming in rep.
Mass Effect 2 has ship upgrades. They are recommended by crew members, may or may not improve your standing with said crew members, and don't do anything because you always fight on foot. Until the cut scene during the game-ending mission. If you didn't upgrade your ship, it gets shot up and you lose several crew members. Surprise!
It also has model ships for your quarters, and fish for your aquarium.
In Harvest Moon you can get upgrades and items for your house as rewards from festivals and events.
Countless quests in EverQuest 2 reward you with house items.
In the flash game Villainous, getting achievements earns various decorations for a throne room, including a pile of treasure and a throne.
The first Mega Man ZX game rewards you for high-level boss victories with models of the bosses, all kept in Prairie's room. The second gives you medals for this, but they still live in their own room, and there are other rooms around the base that you can fill with sidequest items.
Enemies in Wizard 101 and its sister game Pirate 101 can sometimes drop items that can be used to decorate the player's room or house.
The Hatsune Miku Project Diva games with the Diva Room feature allow you to earn new room themes and furniture pieces through performance.
Doki-Doki Universe has decorations as some of the rewards you can get for completing its goals.
Star Wars: Galaxies did this frequently. That said, player housing was one of its most popular features, so there were seldom any complaints.
Star Wars: The Old Republic allows player to purchase apartments as of the "Galactic Strongholds" update and while many pieces of equipment are purchaseable, raids and operations can give specific trophies to adorn your apartment.
Might and Magic : Clouds of Xeen stocks some of its later dungeons with "King's Megacredits," which are used to purchase land, build a castle, and order improvements to the building. Only the final improvement really matters, as the workers digging your new dungeon break into an old cavern holding the all-important Sword of Xeen.
The Elder Scrolls III: Morrowind has strongholds that are built for you for each of the three Great Houses (which one you get depend on which House you join), and which are then upgraded in two phases. The final phase requires reaching the third-highest ranknote In terms of game mechanics. For the Hlaalu and Redoran the third highest rank in the mechanics is the third highest rank in the lore, but the Telvanni's second-highest rank isn't actually a rank, it's a declaration of intent to become the leader of the House in the faction to be started.
In Xenoblade Chronicles X, leveling up your BLADE rank not only gives you color and lighting variations for your barracks, but also gives you hologram projectors, where you can then earn holograms by completing certain quests or fighting certain monsters. Also, you can earn decorative pets to add to your barracks by completing certain quests. Multiplayer quests start in the team leader/originator's barracks, so there's an element of showing off there.