Now that we can send dwarves out to raid other sites, and now that trade routes are a world-gen thing, it makes sense to be able to send traders to other people. I feel that this would be a good place to add player-made trade routes, outgoing caravans, and wagons. Some thoughts on how it would work:
1. To create a caravan (and for larger raids or even military campaigns), you would need to build wagons at an appropriate workshop.
2. Player-owned wagons could be loaded and unloaded like a depot.
3. Players could also order a player-owned wagon to be moved to a specific point on the map, which would allow them to be used for transporting goods internally (although that's not their main purpose.)
4. Wagons can be assigned to outgoing expeditions. In that case, everything in the wagon is available to the expedition. Larger or longer expeditions could require food and might need wagons to supply it; raiders could bring back more if they have wagons to carry it.
5. You could send wagons on 'trade' expeditions, or set up recurring ones. In this case, you set what types of goods you want the wagon to buy (like a trade agreement), load it up, and send it off with some dwarves. The dwarves sent with it would handle the trading (and the highest trading-relevant skills among them would obviously be important.) Dwarves with particular likes or dislikes may end up buying stuff you didn't request, though SELF_CONTROL and DUTIFULNESS can reduce the risk of this. Dwarves who are easily SWAYED_BY_EMOTIONS and with high WASTEFULNESS or IMMODERATION may end up buying more unnecessary stuff, too.
6. Dwarves could be sent to trade without wagons, but wouldn't be able to carry as much.
7. In addition to trading, this could also be used to offer tribute.
8. Most importantly, the player should be able to establish long-term trade-routes. First, you'd send a diplomat to whoever you want to trade with to agree to it; then, you'd assign a depot to that trade route, as well as some dwarves and wagons and a list of desired goods. A depot assigned to a trade route will automatically send any items the player places in that depot outward along the trade route, and will automatically receive incoming caravans and trade with them according to its list of desired goods, without further player involvement. This would allow players to set up ongoing trades that don't require constant attention or micromanagement, while letting the player create trade routes that could be meaningful to other game systems or if the fortress is retired. Players could check in the depot to see items currently offered there by the other civilization if they want to manually force a specific trade, but otherwise it would all be handled by the trade route agreement.