That's a thing I wanted to have for years now. In Vampire: The Masquerade - Bloodlines, the city created a great atmosphere but it was pretty empty. Then there were games like GTA 4, huge cities, lots of traffic but still an empty facade. The problem is, that a living city with buildings that are not just facade will cost a lot of resources. Even in the WItcher games, not all buildings were enterable and most of them looked the same. You can do it procedural but this will end up in a boring repeatable result. Maybe a mixture of hand-modeled areas and generated areas would do it. But then there is still the problem that to much space to explore will end up in dull repetition.
If you could enter almost every building, than you'll lose the interest in doing this will decrease. If the worth of what you'll find in one place is the same as of things in (lots of) other places, that everything together is worth almost nothing. If there are a few places where you can find better things than in the others, than you have still the dull work on looking for that.
Best example are the chests in TW3, hundreds of them, almost the same crap in them, but maybe you could find recipes. To get all recipes you have to look in almost all of them.
Another example is vanilla Skyrim. A lot of holes, caves, caverns, etc. But the loot is almost the same, this makes it less and less interesting (the fact that you can craft everything that you can find, increases the problem even more). Of course there are quests in some places and also finding unique stuff (connected with quests) and of course the beauty of the places and their background story.
