NeverWinter Nights had a castle that you could own. You would hire NPC's on your adventures to repair/upgrade the place. It was a very cool idea but they did not take it nearly far enough.

My thought was that a character should have an armory where all of the weapons/armor/artifacts that are collected through whatever means are kept. Imagine going on a long hard quest for an artifact of immense power. It is the focus of your attention for hours of gameplay. You direct all of your resources into obtaining said artifact and once the quest is over you have an icon in your inventory to show for it. If you had an armory in your castle where you could go and see what all of your effort had bought it would not be so anti-climactic. You could see your items and maybe even interact with them. Click on a weapon and get a 3D view of the sharp shiny blade. Or maybe a cut scene of when you aquired it by slaying a powerful enemy.

Expanding on the idea, you could have a throne room where you talk to advisors, entertain dignitaries, and dole out justice. A treasury where you could see piles of gold and gems and various other spoils accumulate. A library that you need to visit in order to research and learn new spells. Training facilities etc. etc.

In short, I think I am trying to say that I want to feel rewarded beyond a purple or gray item heading. When I defeat an enemy I want something of value, at least in a game world sense. I want my enemy to kneel, or land title, a relic, some of his troops, his loyalty, his daughter to marry my nephew. There are a million opportunities in the game to make it feel as if I am winning without ever getting to the endgame.