This is an Idea I have for helping to revitalize trade. Consider the cart and its limited space, and then consider something like a cart, but is run by horses, could be ridden by people like boats, could be slept in like a leanto, and has enough space for double what a cart could carry. This way, these caravans could be grouped together by enterprising drifters who travel from city to city to trade and deliver goods or letters. They could also be used as protection, makeshift palisades, by simply driving the caravans into a circle formation like in those old west movies. This is to better protect the traders from unscrupulous bandits and brigands who wish to raid their riches.
This provides us with ample opportunities with the professions of mercenary, body guard, merchants, traveling carpenters, traveling cooks, and so on and so forth. I've been thinking on a way to develop this further, and I've touched some ideas which could be down with just a cart and not the caravan. The idea is called Trade Guild and I'll post it up some time.