So the biggest hole in game play I've found, as a new player, is trading goods you didn't make.
I've yet to be forced, other than in the first year, to NEED to produce goods for trading. I'm overflowing with (narrow *.*) (giant *.*) and (small *.*) goods taken from goblins.
I typically have 100+ pieces valued at less than 100, 20 to 30 pieces valued at over 700, and 10 or so valued at over 1,000.
And the goblins...they just never stop bringing their best duds to the fight.
I never pay attention to trader requests because....why bother? They're going to take it, and they're gong to like it.
"What's that? Need booze and meat for the Mountainhome? Here, have some (narrow cave spider silk socks) instead. Yes, I was very glad to do business with you too."
So trading seems like kind of bust except for the value of doing it "right" because it's fun for you. And there's nothing wrong with that. But it would help make trading a more tactical/strategic activity of your fortress if you didn't have so much free wealth to fall back on.
It would also help people (like me) shift inventory. When the trader comes, I always flag the foreign items farthest away from the depot for trading. Most of my trader time therefore is about forcing all my dwarves to move goods they've been ignoring all year.
The end result is that my stockpiles fill up with hundreds of items I've been making to skill up. And then it takes me two seasons for trading it all away to make space....and my ultimate reason is space management, not profit. And I get a distinctly sick feeling when I let the trader make 400% profit because I need to get rid of crap.
So, some suggestions and considerations-
-Limit how much gobbos bring with them. This one probably won't fly. DF likes the super realism of every guy out there has stuff and is usually fully dressed. Like most other RPGs though, this leads to inflation and the devaluation of pretty much everything in game, ala Morrowind and Oblivion.
-Add a modifier for [FOREIGN] goods that slashes their value up to 50% or more. If your dwarves were actively stealing or pillaging someone else for goods, I could see [FOREIGN] flagged items deserving their full value. But as it is, your dwarves are basically bottom feeding and the Mountainhome et. al just gobble it up.
-Devalue any item that was gained through combat. Blood/vomit tags already do this, severely, and I think the [COMBAT_USED] tag could do the same thing, just by a smaller amount. (Say, a 25% reduction in base value.)
I think this is the best solution however
-Add caravan preferences and limits to what goods they want to take. Caravans already have a weight limitation, why do they not also have preferences? And I don't mean that seeing wood products make elves shed a single tear. That's a start, but it's more a prohibitive thing than an actual economic consideration.
In real life, if you try to offload a bunch of crap on a merchant, at some point they go "Hey, I can't sell all this shit! No deal!" Caravans should behave the same way. They should be willing to take unlimited amounts of the goods they requested last season. They should only be willing to take a certain amount of any other good based on: a) how valuable it is to them at that time and b) how much of the item you are currently offering.
With these changes, planning a true export economy for your fortress would actually matter. As it is, you're mostly concerned with your fortress' internal economy, and the export economy usually just helps supplement it. (Food, booze, raw materials.)
Thoughts?
The cut off for a lot of this is something that would need to be tested. But, short of buying masterwork goods from the trader (and who the hell honestly does that?) if you're putting up 5k (20k when I'm feeling saucy) worth of trades with objects you didn't actually make...that's too much.
(And as an aside....I almost never worry about iron for the same reason. Goblins provide me, on a yearly basis, all the iron I could possibly need.)