There does exist a concept that is quite old... I don't know if there is an actual name for it, but it does involve the Sims. I certainly knew about the concept before the Sims came out but it existing doesn't hurt the idea. To visualize the idea in how it functions, you can think of how gameplay in Katamari games works. In Katamari, you will realise quickly that there is only one level (for the most part) in that game, and if you were to progress through the game, you can actually start at the small level and move up to the big level. In between there are different phases or layers you can progress through. To relate this to Maxis games is to add links to all of those different layers.
The original idea comes from having some game that would allow people to play any of the different layers while the unplayed layers (or if it were multiplayer, the layers played by unseen players) would still exist and everything would interact. Think about these games:
- SimCity/SimCity 2000
- SimEarth
- SimTown
- SimCopter
- Streets of SimCity
- SimAnt
- SimFarm
- The Sims
Now try to imagine a single game that includes all of those together. This is the master game idea and Maxis did somewhat attempt some compatibility between those games, especially making it so you could use SimCity 2000 maps in SimCopter and Streets of SimCity.
Current game development seems very far away from this type of thing. We still haven't seen any real attempts at hyper-real environments... by that I don't mean graphics but functionality. Despite the advancements in technology, we still have games with doors that cannot be opened and vehicles that cannot be driven. We still have games with static objects that are just there for decoration and cannot be used or interacted with in any way.
So I think the complete game is still very far away.