I think Civ in general really needs to flesh out economics (a huge influence in history which is dumbed down to tax rate and owning/not owning resources in Civ). The history of money in particular is fascinating. I think FFH is the only really active mod (the one I play the most as well) that could implement something like this.
INFLATION: the amount you lose (vanishes) from your treasury income each turn. Instead of steadily increasing with time it depends on your choices of policies. Part of it will be random each turn but the probability distribution of the effect will depend on currency and the change will be multiplied/divided by other choices selected on the Monetary screen.
Economy here means the total amount of commerce/gold produced in your empire each turn. And when I mention other civs' inflation, the idea is that inflation can carry across trade routes and effect other civs.
For example if I had a large number of trade routes (worth 20% of my turn Economy) with the Calabim and their inflation was 10% per turn then my total average inflation would be 80%*(own inflation) + 20%*(10%). If I didn't have a currency of my own at all and used the Calabims' as legal tender then I would recieve all of their inflation (and be dependent on their policies - nice vassalage idea).
Finally there is no longer simply "Gold" as the universal currency, there can be as many currencies as there civs (but generally fewer), which will start out at the same value. The game will track the relative values of currencies as time goes on depending on the inflation of each.
Options on the Monetary screen:
Currency (flavor option, mostly)
Pull-down menus,
[Currency type name] [Your civ] [denomination, eg, crown/shilling/shield/talent/ecu/denarius, etc, just flavor]
Eg, the "Gold" "Kuriotate" "Dragon"
Or if I had a currency agreement with the Bannor and agreed to use Silver, the "Silver" "Kuriotate/Bannor" "Crown"
If you ever get to a ridiculous situation of having 1,000 of your currency to each
you will be able to click a button here to convert 1,000-to-1 into a new currency and get "New" added before the denomination... "500 New Gold Kuriotate Dragons"... and a reputation hit perhaps.
Currency type (pull-down menu):
Gold coins
Silver coins
Bronze (copper)
Gems
Stone/wood crafting
Barter
special: Paper (after discovering high-level tech/civic)
Each type has a different effect on inflation. Some are inherently more stable than others (Gold is more stable than Silver and Silver more than Bronze)
Additionally the resources you have will have an impact on how they affect your economy. Gold is rare and having one Gold mine should be enough to properly supply a medium five city civ but past that inflation effects (inflation/deflation) will be multiplied. In comparison one silver mine will be able to supply currency for more total commerce and each resource has an optimum amount of commerce.
In other words Gold will be the best if you have enough of it, Silver for large empires without enough precious metals and Barter for those without much economy to speak of and for who trade is not important.
Finally trade of resources becomes more important. Trading Gold will become a lot more important, though it won't be just a happiness created resource which you have or don't, civs without enough precious metals and who want to be independent will be prepared to give a huge amount for it.
The amount of economy that can be covered by each resource unit should decrease. So 1 resource might cover 200 commerce, 2 resources would cover 350 and 3 covers 450 etc. So civs with just one monetary resource should still be self-sufficent in the beginning and happening to have four gold mines clustered in the same place doesn't mean you'll control half the world economy.
Legal tender (tick boxes)
Only own - if you can supply your own currency from resources this would be the best option, you won't be affected by other civs' inflation
OR
Civilization [X] (list of civs whose currencies you accept for payment of tax)
Accepting other civs will increase trade routes (by +1 for each) but increases the effect of inflation.
Minting (pull down menu + button)
Option 1 - stability (create new currency at same rate
grows/shrinks - least inflation)
Option 2 - growth - produce more currency than
grows, will cause inflation but your coffers will fill up in the short term
Option 3 - austerity, less currency produced for
, reduces inflation but treasury grows slower
Create X amount (button) - injects an amount worth up to X% of your total turn
(% dependent on tech level) into your treasury, big inflation hit and potentially unhappiness in cities (depending on government civic, Republics would get the most)
All minting options will be greyed out if you join a Single Currency (Overcouncil) in which case it will be at option 1 (stability) as default... unless perhaps if the Overcouncil can vote on increasing/decreasing it.
Interest (requires tech/civic, "national banking") (pull-down menu)
High
Medium
Low
None
A certain proportion of your current treasury (dependent on option) becomes invisible/unusable (but still there for calculation purposes) but gains you a certain amount of interest (5%) every turn.
If other civs use your money as legal tender then for calculations purposes you would get that interest on the value of trade routes they have with you.
Other civs using your currency would be a good Capitulation/Vassalage demand
Agreements
Here you see the currency agreements you have with other civs. If you selected single currency in Overcouncil resolutions for example. You can also choose fixed exchange rate with certain other civs which will increase trade (+2 routes?) and reduce the inflation effect from that civ. However if you signed a fixed exchange rate with a civ whose currency goes into hyperinflation your exchange rate with all other civs will of course be affected until you cancel the agreement.
In the case of single currencies, the resources of both Civs will be combined to calculate currency coverage (eg, for a gold currency if they have any gold it will be added)
Borrowing
Shows your debts and interest paid per turn
You can borrow from other civs and also from guilds (if they have been created) specified amounts in a given currency. For simplicity the interest rate could be fixed at say 10% (15% for guilds). Guilds would be able to lend in your own currency if you have one established in a city in your realm.
[BUTTON]: Default. Cancel your loan. Should cause war in most cases if another civ, or vanishing of that Guild (causing unhappiness in the city it was in), and deteriorated relations with all other civs. In most cases they would never lend to you again unless you had very good relations together.
What impact should existing civics have?
Foreign Trade: multiplies inflation from civs you trade with
Mercantilism: can still trade with other countries (to a limit) but other Legal Tender options than your own crossed out (so any inflation you get will be all your own fault)
Will any existing wonders have to be changed?
INFLATION: the amount you lose (vanishes) from your treasury income each turn. Instead of steadily increasing with time it depends on your choices of policies. Part of it will be random each turn but the probability distribution of the effect will depend on currency and the change will be multiplied/divided by other choices selected on the Monetary screen.
Economy here means the total amount of commerce/gold produced in your empire each turn. And when I mention other civs' inflation, the idea is that inflation can carry across trade routes and effect other civs.
For example if I had a large number of trade routes (worth 20% of my turn Economy) with the Calabim and their inflation was 10% per turn then my total average inflation would be 80%*(own inflation) + 20%*(10%). If I didn't have a currency of my own at all and used the Calabims' as legal tender then I would recieve all of their inflation (and be dependent on their policies - nice vassalage idea).
Finally there is no longer simply "Gold" as the universal currency, there can be as many currencies as there civs (but generally fewer), which will start out at the same value. The game will track the relative values of currencies as time goes on depending on the inflation of each.
Options on the Monetary screen:
Currency (flavor option, mostly)
Pull-down menus,
[Currency type name] [Your civ] [denomination, eg, crown/shilling/shield/talent/ecu/denarius, etc, just flavor]
Eg, the "Gold" "Kuriotate" "Dragon"
Or if I had a currency agreement with the Bannor and agreed to use Silver, the "Silver" "Kuriotate/Bannor" "Crown"
If you ever get to a ridiculous situation of having 1,000 of your currency to each

Currency type (pull-down menu):
Gold coins
Silver coins
Bronze (copper)
Gems
Stone/wood crafting
Barter
special: Paper (after discovering high-level tech/civic)
Each type has a different effect on inflation. Some are inherently more stable than others (Gold is more stable than Silver and Silver more than Bronze)
Additionally the resources you have will have an impact on how they affect your economy. Gold is rare and having one Gold mine should be enough to properly supply a medium five city civ but past that inflation effects (inflation/deflation) will be multiplied. In comparison one silver mine will be able to supply currency for more total commerce and each resource has an optimum amount of commerce.
In other words Gold will be the best if you have enough of it, Silver for large empires without enough precious metals and Barter for those without much economy to speak of and for who trade is not important.
Finally trade of resources becomes more important. Trading Gold will become a lot more important, though it won't be just a happiness created resource which you have or don't, civs without enough precious metals and who want to be independent will be prepared to give a huge amount for it.
The amount of economy that can be covered by each resource unit should decrease. So 1 resource might cover 200 commerce, 2 resources would cover 350 and 3 covers 450 etc. So civs with just one monetary resource should still be self-sufficent in the beginning and happening to have four gold mines clustered in the same place doesn't mean you'll control half the world economy.
Legal tender (tick boxes)
Only own - if you can supply your own currency from resources this would be the best option, you won't be affected by other civs' inflation
OR
Civilization [X] (list of civs whose currencies you accept for payment of tax)
Accepting other civs will increase trade routes (by +1 for each) but increases the effect of inflation.
Minting (pull down menu + button)
Option 1 - stability (create new currency at same rate

Option 2 - growth - produce more currency than

Option 3 - austerity, less currency produced for

Create X amount (button) - injects an amount worth up to X% of your total turn

All minting options will be greyed out if you join a Single Currency (Overcouncil) in which case it will be at option 1 (stability) as default... unless perhaps if the Overcouncil can vote on increasing/decreasing it.
Interest (requires tech/civic, "national banking") (pull-down menu)
High
Medium
Low
None
A certain proportion of your current treasury (dependent on option) becomes invisible/unusable (but still there for calculation purposes) but gains you a certain amount of interest (5%) every turn.
If other civs use your money as legal tender then for calculations purposes you would get that interest on the value of trade routes they have with you.
Other civs using your currency would be a good Capitulation/Vassalage demand
Agreements
Here you see the currency agreements you have with other civs. If you selected single currency in Overcouncil resolutions for example. You can also choose fixed exchange rate with certain other civs which will increase trade (+2 routes?) and reduce the inflation effect from that civ. However if you signed a fixed exchange rate with a civ whose currency goes into hyperinflation your exchange rate with all other civs will of course be affected until you cancel the agreement.
In the case of single currencies, the resources of both Civs will be combined to calculate currency coverage (eg, for a gold currency if they have any gold it will be added)
Borrowing
Shows your debts and interest paid per turn
You can borrow from other civs and also from guilds (if they have been created) specified amounts in a given currency. For simplicity the interest rate could be fixed at say 10% (15% for guilds). Guilds would be able to lend in your own currency if you have one established in a city in your realm.
[BUTTON]: Default. Cancel your loan. Should cause war in most cases if another civ, or vanishing of that Guild (causing unhappiness in the city it was in), and deteriorated relations with all other civs. In most cases they would never lend to you again unless you had very good relations together.
What impact should existing civics have?
Foreign Trade: multiplies inflation from civs you trade with
Mercantilism: can still trade with other countries (to a limit) but other Legal Tender options than your own crossed out (so any inflation you get will be all your own fault)
Will any existing wonders have to be changed?