We've decided to sub divide Tier 1 into sections of city. This is due to how the Trident3d editor will handle the construction of the city. With 10s of millions of polygons planned in the city, it will be difficult for any tool to handle the entire city at one time. The game engine can handle it because of LOD systems, where the editor has to render every poly all the time. So we're keeping each section of the city to about 500,000 polygons of static geometry.
This is also led to the idea of City Districts. Since we were dividing the city up anyway, we decided to make the divisions based on the Districts of the city. A District is something like: Merchant Disrict (for shopping), or the Entertainment District (where players waiting for friends to come into the game can go for some in game entertainment, like gambling, dancehalls, zoo, fortune telling, etc.)