The problem? Changing the model to a constant stream of traders independantly bringing goods to set up shop breaks the regular seige model that is part of the 'fun' of the game. Either the sieges don't happen, or the traders can't get in for number of goblins constantly marching in.
I think the whole siege/trade dynamic needs rebalancing anyways. Personally I'd like to see traders gravitate toward the metropolises; they'll do the traveling salesman thing out to the villages, but probably base themselves in the capitol where they can trade with other merchants. Trading should be a vital part of survival and prosperity, so it's not a trifle when goblins show up and cut off your supply lines.
I also think goblins should raid more often but siege less. Where you place your fort relative to claimed territory should matter a lot more than it does; you shouldn't expect to survive -- or even arrive -- if you try to start a fort in the middle of someone else's land. On the other hand, if you're in the middle of a bunch of other forts, you probably don't have to worry about any large-scale invasions (unless the goblins take some of the other forts protecting you). This gets to picking the type of fort: if you want to sim-city or megaconstruct, do it at the heart of your civ's territory. If you want to fight goblins, start a border fort.
On a related note, I want the days of finding the perfect site to end, but as-is it's pretty hard to get anything done unless you go somewhere with everything you need. I think improved trade (i.e. caravan arc) would help that a lot; another thing that would help is Retire Fortress so that you can e.g. build a mining settlement to supply you with bars in your fort on a magma pipe; you'd forge weapons and armor there before sending them to the capitol or the front.
This post is strong evidence for Granite's Law.