Well, a Second Life-type game would require, apart from the graphics engine, a modular SDK and purchase, IMO.
In order to have a hi-fidelity "life simulator" you'd need:
-The base game with all the modules (physics, weather, aerodynamics, hydrodynamics e.t.c.)
-The various modules that can be purchased, coming with the relevant SDK
--I.e. the base module comes with the ability to walk, drive and interact.
--Then come the sea, flying, professional driver's (bus, cab, truck driver) and possible military modules that would work in conjuction with the previous ones.
--Perhaps a builder module for the wannabe engineers, allowing you to build roads, bridges, runways, harbours etc. Of course, the construction module would provide costs in game currency which will have to be payed by the "constructor" or his clients.
All of these would be unified under a single graphics+weather+Multiplayer+physics engine.
This way, you could possibly have a world where anyone can walk/drive and build some basic structures. Then some specialists would have to develop the infrastructure (engineer SDK) for game currency. Others would be able to work as transportation professionals or military. Finally, people without the vehicle modules could work in the services area, practically being middlemen that sell accessories (game clothing, houses, vehicles, game currency) to other players, in exchange for game currency.
For the last category, you can even go as far as adding minigames. I.e. for car maintenance, you could be playing a relevant, small puzzle game depending on the severity of the damage to the car, or the servicing type.
Decoratory NPCs could also be used for manual labor, since not many people want to be car engineers or construction workers in videogames. A player could manage the financial aspects of such an enterprise.
Finally, all these actions would require time to be completed. You can't build a block of flats in one week. For game purposes though, you can, but if apartment blocks pop-up every week, there would be no interest in buying or negotiating a price for a flat.
Of course, the developer could supply an already-built world, and together with the above DLC/modules allow the players to mod it further.
