I thought a bit about the way supply and demand would work in an eventual trade overhaul. I'm thinking that perhaps one should look at the way trade goods worked in real life for a pointer, and see how it contrasts with Dwarf Fortress as it stands - not that realism is always the best, but it could be helpful.
Staple Crops
While the list of new edible plants in Dwarf Fortress 2014 is impressive, one notes that among them only a few are really important for people historically, and in the present. These are the staple crops - the big three are wheat, rice and maize, but there's also others such as potato, millet, beans, sorghum, taro etc. They are good for planting on a large scale, and are crucially good for long-term storage - providing a source of food during winter, dry periods, sieges etc. and are better suited for trading - it just makes more sense to trade flour rather than fresh food like cucumbers. Other crops were of course used, but more as a supplement.
There were some exceptions to this. The most notable example is parts of Sub-Saharan Africa, which relied on perennial plants instead. Life was arguably better, but it sustained less people, leading to low-density areas with not a whole lot of government involved - the historical civilizations such as the Mali relied on trade, not taxes. This came to bite Africans in the ass come the colonial age.
While a mundane commodity, it was big money. While the Dutch are more famous for the spice trade, the base of their trade was the grain trade in the Baltic. Places not self-sufficient in food are very reliant on it.
This of course doesn't make a whole lot of sense as things stand now in the game. While a future agriculture overhaul will undoubtedly change things, it's something to keep in mind. From a realistic perspective, cave wheat should the dwarven trademark food, not plump helmets.
Gold (& Silver)
Why is gold so valuable? One can't eat it, sucks as a weapon and weighs a crapton. Its rarity and endurance (and looks) makes it very suitable for a currency, though. If one bases a currency on something like gold, there's one thing to keep in mind - mining gold is essentially printing money. If more gold seeps out of your civilization than in, one'd got a real problem on one's hands. And while mining gold is extremely lucrative, mine too much and one might run into inflation.
It should be noted that society was much, much less monetized in the past than now, settlements relying on debt for exchange and taxes paid in natura, currency mostly being relegated to war and trade. If one dooesn't have a lot of coins in their fortress, problems with caravans and mercenaries is logical, the fortress crumbling because nobody can buy or sell anything within the fortress is not.
Salt
"Worth one's salt", "salt of the earth", "take with a grain of salt" are expressions, and the word salary is derived from salt. Before canning and refrigeration, salt was the primary method of preserving food. A Russian man who lived isolated in Siberia with his family said he didn't miss much except salt, describing life without it as "pure torture". It goes without saying that salt was very, very valuable.
As things stand in-game, this wouldn't make a lot of sense. Anything in a food stockpile with cats essentially has eternal shelf-life, unless it's vermin fish. Having a food overhaul before the economy would be a really good idea.
Hunting & Trapping
Animals in the wild have been sought for their products since time immemorial. Meat is of course good for survival, but the others parts are more important in economic terms, such as fur, ivory and blubber.
It would be a more risky way of doing business - if a fortress hunts the local merman population to extinction in the hunt for bone, that's bad. If mermen thrive despite hunting, but a goblin civilization suddenly think they're lame, that's also bad. The first is somewhat modelled in-game, the latter not. And of course, the importance of blubber, such as from whales, is lost without a lighting system.
Fishing
The ocean used to be much more sprawling with life - reading old ship logs, one can hardly believe it. Back then it was a really abundant source of food, beating even staple crops in some places. And despite their shelf-life, fish etc. were a common trade good. If fished sustainably, the fish population should be able to recuperate. However, short-term catching more is better, even if stocks suffer in the long-term, and so it goes. It would be interesting if this could be modelled for worlds with a long history, and be an impetus for making more colonies. Depending on culture or religion, fish could be awesome or un-kosher.
Slaves
Not the most moral of "commodities", it's nevertheless kind of a big deal. Currently only humans and goblins do slavery. It would be interesting if the kind of slaves wanted changed, and making slaves became a goal. A human civilization might value elf scholars as teachers, and a goblin civilization young elf females for, er, entertainment.
If a gob civ comes across fertile land, they might decide to make a sugar plantation. First they use local animal people, but they die due to disease and overworking. So they try to find other sources - perhaps they raid elves for war prisoners, buy slaves from humans, or change the law to make slaves from their own population.
Iron, Copper, other strategic metals
Perhaps if there's not a lot of iron around, tin (for copper) is more valuable, and if it is, maybe tin is used more for decorative objects. Sites without a lot of heat sources should not want a lot of raw ore, but desire finished objects more. I don't have a lot of thoughts on the matter that hasn't been said (or is in-game) already at the moment.
Cloth
There are a few things that could make a difference as to how much cloth is worth: If it's cotton only growable in certain places, that could be more valuable (overhaul agriculture again). Perhaps certain civilizations have a certain style that is more fashionable. Demand for cloth could vary a lot depending on how much sailing is done - archipelago would be better for cloth growers than pangaea.
Dye (& Paint?)
Dyes were big money, with the notable example of the trade empire Carthage having dye as its largest source of profit. Lime green could be valuable since it's only found in a type of caterpillar that live in evil forests, a clear status symbol. Midnight blue meanwhile isn't worth a whole lot, but it could make one's dwarves happier.
Precious Stuff
Gems, amber, pearls, shells etc., things that don't have a whole lot of practical value, but are still valuable as decoration or status symbols. Somewhat represented in-game, but currently there's not a lot of need for them as one can just make a thousand rock trumpets and sell that, anywhere in the world.
Stone
Given that stone is practically everywhere, stone mugs should not be so valuable, nor should an export industry be built around them. Yet, they were important as trade goods, though not so much finished goods - some places were really good quarry locations, profitable if buildings or monuments were built with them. Of course, their weight means that the places to trade them with should be limited. I'm not sure how to model quarries in-game.
Marble
Currently, marble is valuable as a flux stone, but it was also valuable for other reasons - they were great as building materials and for sculpting, and should sell well if that kind of thing is wanted. As building material, it should not sell well in cold regions due to its lousy heat retaining properties.
Peat & Coal
Energy is important for civilization, that much is obvious. One important aspect not focused on a lot though; the difference between kinetic energy, derived from sources like muscle, wind and water - and heat energy, derived from wood and fossil fuels (and magma). Historically, the latter is much more valuable. In-game only a few things need fuel, but in real life there's the two crucial ones of preparing food and heating buildings. If a site has a whole lot of coal or peat, it makes sense for it to be a manufacturing center, especially since they're hard to trade due to weight issues. Magma forges are a spanner in the works, though.
Wood
A really good material, not only for the heat it provides, but the myriad of other uses, such as for ships, buildings, ammunition etc. Trees are really common, though, and their weight gives some restrictions for profitability.
Since trees grow quite slowly, deforestation could be a real problem for individual sites, and even civilizations if the history is long enough. It could be different depending on civs' attitude to nature.
Fortresses in a heavily forested region could have lumberjacking as an industry if demand is high enough. An archipelago world with long history could be golden. There's also difference between log and log - full-grown oak is better than barely grown birch.
Elves' ability to magically grow wood and their incessant attempts to stop tree-felling is obviously a spanner in the works here.
Beverages
Drinks derived from plants such as grapes, tea, coffee and cocoa were profitable, and still are. Water quality kind of sucked for the most part, so these were important from both taste and hygiene point of view.
These crops only grow in certain places (there's that need for an agriculture overhaul again) and price depends on the quantity of slaves and how much a civilization values the beverage. Dwarves are all alcoholics, so there's that, obviously.
Spices, sugar
Without refrigeration and canning it was more difficult keepings things fresh, as previously mentioned. Without the cargo shipping of today bringing in harvests across the world, less fresh products were available. This meant food tended to taste bad, really bad - a little, or preferably a lot, of spice or sugar makes it better. The spice trade was the primary motivation for Europeans finding another way to India.
While they don't have the preservation properties of salt, taste is better and can only be grown in certain climates (agriculture overhaul!)
For the nobility and rich merchants across the world, their demand is nearly unfillable, and it's a pretty safe and stable way to make money. Of course, there's labour costs...
Tobacco and other drugs
Certain substances have addictive effects. Getting a fresh supply to the drug dependent is a way to make a living. The caveat of only growing in certain places apply (agriculture overhaul!), and demand could vary a lot depending on religious and ideological taboos. Civs could make it a goal to try to get sites addicted to their special merchandise.
"Chinaware"
A lot of things are valuable more because of difference in knowledge rather than resource availability. Porcelain was valuable not because of rock crystals, but because only the Chinese knew how to make it, and so was silk. One could make more profit selling binoculars to animal people rather than dwarves.
Commodities obviously become less valuable if rivals find out the trade secret and start competing industries. It would be interesting if it could be modelled in-game.
That's all I can think of for now.