"I think I have a simple idea for how the trading could affect gameplay. If the players, instead of simply getting SCs got points for SCs*value of currency, then they would want their currency to do well, and would therefore negotiate with the traders to get their currency doing well. That way, the trading and playing would be linked."
However a strong currnecy doesn't equal a strong economy, when your economy is weak, you want your currency to become weaker, and as Amby pointed out, when the currency is weak, paying off debts is easier.
If we wanted to, we could change the game to have two resources rather then 1:
Supplies and Money.
Each army and fleet would require 1 unit of supplies and $20,000 to maintain.
Each army and fleet would require 1 unit of supplies and $75,000 to build.
(The supply of units will be covered by the game, however money will be covered by us. If a government lacks the funds to build new units but has the supplies, said governmnet must waive thier build. If the government lacks the funds to maintain each unit, but can't disband units because they still have supplies, they must announce at the begining of the year a unit which will be 'under funded' an under supplied unit must enter holds for each turn that year and cannot support yours or allied units. If an investor wants, he can choose to fund that unit, in which he will PM orders for that unit to the player nation. Or an ally to said nation may choose to fund it for him. However if its an allied nation, the player can choose to accept the transfer of the unit)
If more then half or your units are under funded, there will be a military coup, and said player must enter all holds for the rest of the game (as if he left the game).
Governments gain $30,000 for each home supply center they have, $15,000 for each nuetral supply center, and $5000 for each enemy home supply center.
In other words, if they ever want to build units beyond their additional 3 armies (or 4 in the case of Russia), they would have to borrow money (if they get 3 builds, they can only build one army without borrowing mony). The most common form of borrowing money for funding wars is selling victory bonds.
Each victory bound would be sold for $1,000, and would provide anual source of interest (The interest rate being calcuated based on how many people are willing to buy the bonds), before maturing and the government pays you back. If the governemnt either goes bankrupt, or looses the war, it will not pay back its investors, if the government wins, it will pay back the investors.
At the end of the game, each country will gain $150,000 for every supply center they control, regardless of how the game ends (solo or draw). They will use that money to pay back their investors and all thier debts.
This could actually work, because it incorporates the game mechanics of real diplomacy, and adds the finance side.