Two problems:
1. As noted, partially filling orders would be nice. My trader has a big pile of profit staring him in the face.... and can't access it, because partial orders don't work.
2. Unprofitable trade needs to be possible. Sometimes I stand to gain from an unprofitable trade; such as if it manages to fuel money into a region in which I have a significant oath. Let's say I make a 200 gold transaction, and lose 20 gold on it. If the income of that region rises 200 gold, and I have an oath for 30%, I just gained considerably. It's not mere altruistic nationalism: trading at a loss is a valuable tool for reaping personal rewards and for making political allies.
With both of those, in my own experience, a significant amount of new trading could open up.
