Buying your currency back (taking it out of circulation) would be done as with all other trade goods, and would cost as much in valueTM as they did before. You would gain nothing in this process, as the only way to buy coins back would be to mint new coins or change the value, both of which have other repercussions, as they do in real life.
WRONG!
You've also got me backwards - buying back currency would be done with goods, and would fill the function of A) People redeeming your currency for backing goods B) You removing your currency from circulation, and prolly melting it down/destroying it, so you don't have to back it.
If traders brought your coinage, then that might be because you're doing well. Eventually everyone might be using your coins, which is no big problem (think of EVERY COUNTRY ON EARTH. The American dollar is accepted almost anywhere, why should DF be any different? If you manage to be a global superpower, you deserve to have the rewards.
Bringing your coinage to buy things is functionally the same as bringing it to redeem it's value. They
could bring it as a 'trade good', in which case you wouldn't have to buy it or face consequences, but that's mostly not what I was talking about(unless you requested it for the purposes of removing it for circulation/whatever).
In general, however, you'd have no use for your coinage, so traders bringing it to trade with probably isn't something you want - it's an export.
Backing goods checks would be performed by special liasons who would appear from a civ or town or whatever, bringing back all the coins that they wanted redeemed, and you would have to trade them their goods. I think that a vast wagon should come along as well, so that they WILL be able to take any goods you offer them. Maybe only actual goods should be allowed, so hauling 14 tonnes of stone in backing goods wouldn't be allowed. Having separate events for backing goods checks is essential.
So, each civ can now send another caravan, except these ones we
have to trade with or our currency and reputation is screwed.
My god, man, dealing with three a year can already be a pain, with all the hauling and the dwarf supposed to meet with them/trade/whatever going off to haul/eat/drink/sleep/on break! Special caravans will be nothing but a burden.
I'm also not convinced having them as separate events is essential - I see no point, and several reasons not to. It adds nothing to the game.
Rethinking about the mountainhome minting has brought up a better solution. Fuck it. People wouldn't trust your currency after only a year or two, but oh well. Other fortresses crumble after three or four years, but oh well. Screw this bit of realism, let's go with what's fun.
I'm really not happy with the idea of royal coinage and fortress coinage. I think every fortress/site should have it's own coins to trade, and they all kinda compete for dominance. I think that wagon sizes is dealt with already by how much you trade, and similarly...
Royal coinage is an implementation of your idea for asset-backed currency being a king-based reward, with me suggesting the addition of duration enabling access.
It should, I think, have some prerequisites. The king is a good one, as it adds legitimacy, but there are a few other handy points.
The wagon size is less logic and more player-incentive based. As is, your suggestions seem to boil down to a horribly complicated, laborious and fragile way to moderately inflate your available trade value, with no real incentive to do so.
...and let's be clear, buying out caravans is easy - introducing something so complex that only generates more trade value is virtually pointless. It adds very little to the player experience...
Rather, allowing the player to better increase caravan size via coin use(and selling trade goods/barter use will still increase it under my suggestions, just not as much) is intended to provide a solid, innate incentive and benefit to using both forms of currency beyond "style" - and something this complex needs more than that to justify itself.
...if people trust your coinage, that will boost the wagon train because they know that you are a strong trading partner and that the possibility of good trade is high. This implementation seems simpler and sleeker.
You really know how to pick your battles, don't you? On everything else you've opted for cumbersome and needlessly complex.
Not only would that be far more abstract, but you don't get business because people trust your currency, you'd get it because people want what you have to offer(the goods and their cost). Having a strong currency is part of that, as money is a very handy and appealing store of value, but you really seem to have the reasoning backwards.
Still, this is pretty similar to what I proposed in terms of functionality, just shallower and... oddly justified.
I think there should be a pretty constant flow of people wanting to check your currency in the early years, with the peak times being after crises and wars. The thing about asset backed currency is that, yes, it leaves itself open to exploitation (minting more coins than you can back), but every coin minted is a gamble that enough people won't check to collapse your currency. It's all about making people think you have the goods, and persuading them not to all check at once. Exploitation is the very fundamental basis of all established credit systems.
I know what you're talking about, but how you want to implement it in DF, well, it's just cumbersome, impractical, and adds very little to the game.
My final suggestions in my last post is a far more streamlined implementation with logical, optional incentives and good versatility. Add the hinted ability to refuse foreign liaisons the option of checking your vaults(an action that would be abstracted, so they don't have to run down to the vaults and can do the task from the meeting. Like how the book keeper doesn't have to run around to take stock) and you can implement the deception that you want, albeit at the cost of trust when you refuse their request to verify(trust prolly implemented as a limit on the backed-coinage a civ's caravans will accept, be it overall value and/or percentage of value above the coin's innate value(ie, if the Hunams don't trust your currency a whole lot, they might only accept coin denominations 150% their innate value, so coins with an innate value of ☼60 could have their value boosted to ☼90 via backing, and they'd accept that, but stamping a ☼92 denomination would be pushing it too far, even if you have the goods. Atleast until you build up faith in your currency)), but this can be offset by gifts and proving your money reliable over time.
Maybe even an option to bribe the liaison to claim your vaults are wealthier than in reality? Prolly depending of your representative's social skills VS his own and/or personality traits(determining how susceptible he is to bribes and corruption).
Now, I can see you scraping the Dwarf Liaison verify-and-license, or making it optional, at the cost of taking longer to build up trust and thus the size of denominations and the total value of currency you can export with any given caravan. The trust mechanic I mention above would seem a decent balance to prevent excessive
easy abuse while being fairly plausible, I think, while V'n'L allows the honest(or those who can bribe the dwarf liaison
) a faster way to build up trust, with the caveat that suddenly deciding to refuse him would seriously hurt overall trust in your asset-backed, license-minted currency(and if your license-minted currency collapses, it'll have a backlash against your kingdom's government and economy, depending on how much is out there and how big the collapse is).
And of course you always have the option of traditional innate-value coinage - which should become far more popular if you've caused a few economic collapses with previous forts in the same world, as you've undermined overall faith in asset-backed currency in general.
Remember, while it's a neat idea, you don't want to unnecessarily burden the player, and you want to give them a solid reward for doing it(especially given the depth and complexity we're talking about) - extra trade value means little overall(atleast until we have alot more to spend it on - it's too easy to become incredibly wealthy as-is), and tying caravan size to it isn't actually a huge benefit either, but rather the best I could think of.
It needs something more, IMO, but I can't think of something appropriate.
EDIT:
MrFake, the focus here is external trade, not the internal fortress economy. Atleast, that's what my suggestions are aimed towards. As a player, the internal economy isn't something I'm personally fond of.