Couple of upgrades that would be awesome:
1. After your fortress reaches a certain size, dwarves will begin to bring wagons for trade in heavy items like stone and metal. We already know that dwarves can use wagons since starting fortresses use them to carry supplies.
2. Add metal framed wagons. Same as wood wagons but they can carry more weight.
3. At the same time, add lumber, box and flatbed wagons. These are built for the transport of wood, stone, livestock and other large bulky items like furniture. They are heavily reinforced but will only carry specific types of items.
These wagons might show up empty, this allows us to trade away large quantities of things like statues or siege weapon parts that would not fit on normal wagons.
4. The elf "ambassador" also works as a trade liason. You can then request that elves bring certain items over others. (instead of endless rope reed). The elves will tend to request items like metal cages and stone mugs. Basically stuff they cant make themselves or would have to use precious wood.
5. Add sand to the list of items available to trade from dwarves and humans. Seriously, if someone can make a profit selling me bags of sand, they will. While your at it, also add raw gems/glass to the list.
6. Allow dwarves to trade for coins. These coins can be traded back to traders in exchange for goods, leaving more room in the wagons for heavy objects. The traders like this because it allows them to carry home a lot more stuff.
This also means you can avoid giving the traders ridiculous profits when all you have to trade are very expensive items. Ie trading a crossbow for a masterwork roast.
7. Bring back roads. This time around, when you connect your trade depot to a map edge, trade caravans will always use the road and not appear out of the wilderness. Stronger and smoother roads allow the use of heavier wagons, and thus more stuff for you to buy.
This also applies to migrants, liasons and other entities. Its EXTREMELY annoying when you pick a map with extreme cliffs and migrants show up 10 levels above your fortress on a tiny cliff apparantly having walked over 15+ mountains.
8. Allow us to specify the quality of item we want. The default is standard. If you want something special, such as high quality weapons you can request those instead. At the same time you can also specify whether or not you want decorated items. I can't count how many times ive seen items i want but cant afford. Things like exceptional iron crossbows decorated with masterwork star rubies and rose gold...
9. Anything the rest of you have to add.
Thoughts?