We'd need some type of fortress-milestone for sending out our own caravans. Otherwise, we could get into the silly situation where your starting 7 repurposes its wagon/pack animals and sends out your broker to trade. I am not saying this is a good idea... just silly. So we'd want to at least be a town (have a mayor), but I'd rather set it to a barony. Sending out a liason and sponsoring trade routes reeks of nobles. Like captain of the guard, you'd get another position pop up: Trade Minister. S/he'd be like the manager for trade routes. You can select which sites to trade with, and toggle what you want from them while checking what they want in return. I don't know about anyone else, but when the dwarf liason tells me what the mountainhome wants I can't remember that crap. I usually just take a screenshot- its definitely a nightmare if I am juggling forts or haven't played in a while. A place in game to check all of this would be a god-send. From this menu you'd also select the parts needed to build a proper caravan, then mark the items for trade along with either quotas or a priority system on what to trade for. The skill of the broker you put in the wagon seat would ultimately decide your profit margin. This... would also be much simpler if currency was re-implemented. However, then you'd have to question the value of currency from different sites and also the metal-content of said coins... and that gets into currency wars alongside trade-route wars >.> Besides, what would the elves use, wooden coins? Leaves? Psh. Hippies. They don't even like using bones on a species-wide level.
Yes, yes, we can make liasons from other races appear by tweaking the code, but its still for just one site. Going on what GoblinCookie said earlier, we'd be seeking to seize that "first traders advantage" in setting up new routes with untapped sites. We also might end up trading with multiple sites of the same race, so we'd need different liasons for that. Toady also mentioned that he wants little villages of hill-dwarves to pop up around fortresses. While these probably wouldn't need a caravan (or one caravan could visit many), it would still be several distinct sites to keep track of what they need/want (trade minister). We could also send peddlers to them, possibly. Historically, these would be the bread-baskets supporting the main city with food in exchange for manufactured goods. It would be cool if we could completely supply our fort with food/cloth via these peasants, while sending out weapons, crafts, etc in exchange.
As far as setting up a caravan... that wouldn't be so hard. Our base wagon breaks down into just 3 logs, so I'd doubt we'd need more than that + a carpenter to assemble it. We might want some leather to represent the covering and a rope or two to represent the reins, though. Attach the beasts of burden, appoint a trader, attach some guards, and then load her up with goods. Tell them what to buy and where to go. Its already ludicrously easy to over-produce goods, so it shouldn't be hard to load it up with enough junk... er "valuable goods of the highest craft-dorfship" to make it profitable.
The main problem is that trade if fundamentally broken as it is. We need far too little from a caravan and can produce far too much with far too little effort. There'd need to be an over-arching total industry overhaul. Some form of reason to actually keep trade routes up. As it stands, we basically just buy a little extra food and cloth in the first caravan or two. After that, we only pick up metal items to melt down. If you already have iron and flux, you'll likely never want to trade past the first year or so. Strange-moods won't request metals you haven't smelted yet, so you wouldn't need to pad those numbers outside shiggles. Food would need to be harder to make (soil shouldn't be equal and it shouldn't last forever, soil depletion!) or dorfs would need to eat more. Crafting would need to be slowed down in either quantity or quality (paired with higher tier of quality available from trade). It is rather silly that no one seems to be a legendary ANYTHING until they show up at your fort. A year of making rock crafts later and they are legendary! Truly, in the entire 1000+ year history of my world, this is a first. There would need to be some form of supply/demand so we couldn't flood the market with stone trinkets forever. There need to be more supplies from other sites we'd want. Perhaps there could be things like spices and incense that would boost happiness tremendously- luxury goods that only appear rarely. Perhaps trinkets and items from other sites would have an inherent "exotic" value multiplier to them? Toady mentioned having special recipes for cooking that could be randomly generated and become hallmarks of taverns, so perhaps a similar mechanic could be at play for trade. Certain sites become "known" for a trade good. Think Persian Rugs becoming a hot luxury item, but on the dwarf-fortress scale. Medieval brand names, if you will. These would have much-higher values/happiness boosts to them, but only come from specific sites. Again, trade needs a huge boost if we are going to start trading with even more sites. I feel overwhelmed as it is just trading with humans, elves, and dorfs. If I am not in a metal-poor site, I ever ignore them or trade them token goods for the sake of appearances. Jumping that number up to 10 would be superfluous in the extreme for the current game.