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.
1. Lords who want partial trades can already make small offers easily (refresh).
2. Trade offers are
personal offers, so those are bonds that go to the lord or steward, not to the region.
