Hello, first post. So I have an idea.
Stock Market.
Say when a person becomes King of a nation such as the United States, they can issue out a company which sells stocks. The stock market changes based on the sale and trade of game materials. For example "Smithers' Rum" had their stock tripled in the past 24 hours, paying dividends to everyone who invested in "Smither's Rum." Companies such as "Smithers' Rum" can be created by anyone willing to pay 15 million dollars in order to start their own company. Companies can be sold/exchanged and players can own multiple companies and share their company with multiple people.
Knowing the stock market is extremely complex, paragraphs upon paragraphs could be written in order to patch corruption and state the rules. But this is simply the basic overview.