I think trade could be simply regulated through an Economic Advisor. That screen would look a lot like the current Foreign Advisor screen, i.e. a web of lines and portraits(no more animated leader heads). Between each power would be a line for their trade relationship, with barriers listed. You would also see special colored extra lines for FTAs(since they would be formal agreements). Of course any of these deals can be made on a multi-lateral basis.
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Trade Barriers
You could simply set the barrier for trade for a nation. All tariffs are on imports and are recieved by the importer. Also, each trade(the culumulative export from one nation to another) would be the base amount for determining how much of whatever is earned.
Open - The importer earns 100% the base amount in trade arrows(applied to the national city, which is a made up entity to spread the percentages against, happy faces would be distributed where needed then to where they would make WTLKD.). The exporter earns 100% base in gold and 100% base in trade arrows.
Light Tariffs - The importer earns 50% the base amount in trade arrows and 50% in gold. The exporter earns 75% the base in gold and 75% the base in trade arrows.
Heavy Tariffs - The importer earns 100% the base in gold. The exporter earns 50% the base in gold and 50% the base in trade arrows.
Restricted(Specific) - You can specify which resources can be exported.
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The round system effect of trade barriers are this. Suppose Civ A and Civ B have approx. the same base of trade.
With Open
- Tariffs = 100% Trade Arrows
- Exports = 100% Trade Arrows and 100% Gold
- Total = 200% Trade Arrows and 100% Gold
With Light Tariffs
- Tariffs = 50% Trade Arrows and 50% Gold
- Exports = 75% Trade Arrows and 75% Gold
- Total = 125% Trade Arrows and 125% Gold
With Heavy Tariffs
- Exports = 50% Trade Arrows and 50% Gold
- Total = 50% Trade Arrows and 150% Gold
This analysis shows that free-trade overall will benefit more, but that a country trying to make moeny off of heavy tariffs will usually be answered. The only times you would see non-paralell policy would be in a situation similair to mercantilism. The mercantil power has Heavy Tariffs on products from the oppressed power. The oppressed power has Open so the mercantil power earns the most gold and keeps the oppressed power poorer.
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These are not complicated commands and would add a lot to cooperation, considering you coudl generate a lot of gold and reserach from seemingly building.