Well, EVENTUALLY, Toady's going to get around to that Caravan arc, and different locations will have different products and potentially have different goods they export and need to import.
To use Pharoah as an example again, different cities would have arbitrarily enforced products that they could or could not build. Some were obvious - copper veins are needed for getting copper for making bronze weapons, and without copper veins, you have to import your copper. Others were pretty arbitrary, like the game disabling clay pits so you can't dig up clay (which only requires dirt near water) to make pottery, forcing you to import pottery (which is a low-level, and hence, absolutely mandatory good). Other goods might be something like importing a food you can't grow locally, because the number of varieties of food is a luxury stat in that game, and the upper classes are happier/wealthier/more taxable when they are given more exotic foods.
The problem in Dwarf Fortress right now is that almost every fort can create every product. We need some sort of regional specialties... in Improved Farming, I've been talking about making many of the aboveground plants biome-specific, but if they're all just food, then it doesn't really matter.
This might be changable if we have something like alchemy, where different plants are used for different products, and the resulting crops aren't so fungible, or if we have something like mulberry bushes to produce silk, or laquer trees who can be farmed to make laquered wood, or exotic dyes.
Mining for gems is generally something that is fairly regional, and can be done for centuries in real life, but mines get depleted of gems so quickly in this game that they're basically useless as an industry, except for carving glass. We would need to either have some way of having mines regenerate their ores and gems, which would actually help solve the problem of how loose this game is on matters of conservation of matter, or to make more than just one gem be produced from a single wall, but preferably make extracting said gems from the mullock much more time-and-labor intensive. (It would also help to stop making looking for diamonds a complete waste of time. Searching whole layers for ONE diamond?)
Other than that, there's probably going to be some major problems when some resources in the game actually stop being completely infinite. We rely on goblinite now, because goblins and their metal equipment are inexhaustable, but if they come from worldmap locations, that can easily start to change. There may come a time when all the iron in the world is used up (or at least, lying in piles on the corpses of some forgotten battlefield on the worldmap). Even melting stuff down is "lossy" for recovering iron.
IRL, there's ways we can get around that. We can melt down iron and reuse all of it, not just 30% of it. There are far, far more veins of iron ores in the world, or rust dissolved in underground water supplies, or whatever in the real world than in DF.
... Actually, I wonder if we might someday get some ability to extract iron ores out of water supplies like that...