Alright, so I have several ideas:
First, I think Rio would be an excellent location. It is so big that the atmosphere would take elements from almost all GTA games. The downtown could resemble Liberty City, while beach/more coastal areas could resemble the semi-resort areas of Vice City, and the slums and other areas could incorporate elements of San Andreas. Also, the uber-dense 'favela' slums would be awesome to play in, with their insane inclines and windy, dense streets (google 'Rocinha', which is the favela I'm mostly thinking of).
Perhaps more important is how I think the city should be built.
One of the problems, I find, with GTA 4 (and all sandbox games, for that matter), is the detail of the map. To build something like Liberty City, you have to spend tons of time, and their simply isn't the time to spend on small details or interiors, which are so limited in GTA 4 (and generally in all games).
Basically, I think it would be awesome if players could build the city. My idea would go like this:
1. When people buy the game, instead of buying a city, they buy the topography of the city, with few buildings. (obviously the price, considering what you get, would be much lower, though people would pay for the other features) along with all the physics, vehicles, weapons, character models and non-building elements.
2. Next, players would buy points, like Microsoft points, with which they can buy raw materials or templates, with which they can design (or just build template designs) buildings and their interiors, which can be entered (or broken into) in the game.
3. Also, players would pay a monthly subscription fee to download daily updates of the map, which would include the newly-built or changed buildings.
The developers would have to include certain building limitations, like if someone builds on too steep of an incline or uses the wrong materials, the building collapses.
This would not be an MMO. Players would play locally or with a multiplayer like GTA 4.
However, players would be able to pause the game, select a building, and see the information of the builder, and be able to communicate with them.
One issue that would have to be figured out is why players would want to build buildings when others already have, a trend which would result in no buildings being built. To get players to build, they'd be required to have a home (rented or owned), as well as furnished to a certain degree (required: bed, sink, table, etc) so that when players enter, they will have something to look at. Worse buildings, like in the slums, would be cheaper, while nicer buildings, like downtown or beach-front, would be more expensive to build (and thus more expensive to purchase from other players).
Consequently, people could buy property using points off of others, and get around building, if they have the cash. Since these points are actually worth money, this would result in some people having really nice buildings, if they want to spend the money, while others have really bad buildings, because they don't want to pay. However, here you would run into another problem. There would need to be some sort of incentive to have nicer buildings, such as better weapons/vehicles/clothes/etc, different missions, or something else.
When someone pays someone else points for the "deed", that would be exchangeable in real money paid through paypal or something like that. This way, their would be an incentive to sell property like in a real market. People could also rent, which would be cheaper in the short term, though not in the long term, and allow builders to make a profit on building a multi-unit building.
In this scenario missions would also be a problem. While you could just cruise around, killing civilians and prostitutes, missions are still some of the best parts of the games. However, making missions in player-created buildings would be problematic, as they could obviously change. Consequently, the developers would have to construct certain buildings/areas in which missions take place.
So, the idea obviously has a lot of kinks.
Nonetheless, a city created by players, which constantly shifts and changes, would be pretty awesome. Considering the money-side of things, I think it would develop like a read city, with varying degrees of wealth, neighborhoods, etc. This would make the city pretty damn cool to play in.
PS: Elements of Rio's real economic differences could be introduced: Players could have a simple job if they buy a shitty house, or a more complicated, upscale job if they bought a better one. Basically, I think the missions/story-lines should be different based on how much money you spend, with wealthier players dealing with rich people, like in Ballad of Gay Tony, or if you spend little money, the story line is grittier, like in Lost and Damned.
So, anyways, this is just what I'm thinking would make a sick game. I'd sure as hell buy it.
Any feedback?