Mine would be...
1. Indian/Persian CB, rich in music, arts, and of course many things to trade, like certain fruits, spices, and jewels, with enough climates to suit too! I think this would be the most hopeful as a CB.
2. (This is a common suggestion among Citybuilders)
Aztec CB. I don't know how they would avtually sort this one out, as only large cities had roads. I think this would be more suitable to a stronghold style game. (No roads)
But if it was a CB, you could have obsidian, feathers, and a few other rich materials (like gold). You could have cocoa beans, and many wild game too.
3. An African CB. I don't mean loads of little tribal villages, but large trading cities like Timbuktu, Tunis, and maybe even the trading towns in Zimbabwe. Just a suggestion. But again many rich materials, like Gum, Salt, spices, and scrolls of literature. I don't really know whether this will work. As there are limited cities that you could choose from in the north of Africa.
This one has a 'few' problems for a CB...
An Incan CB would be hard to turn into a CB, but some people I know have suggested it, so I will include it. Many Incan cities AND villages had roads, but the style of the game would differ a lot. In many areas, the Incas would live in a cluster of houses on a hilltop, farming below. With small villages, this would be a problem as it would comprise of just a few hilltop houses.
But still, many rich materials to trade. I don't know how this one will work at all, though.
So, what are your ideas?