I would like to see money play a larger role. You could start off by bartering, then progress to gold, fiat currency and, perhaps, blockchain technology. Gameplay could implement a federal reserve, IRS, and treasury. Taxation control could impact happiness. Turning up printing presses to create inflation and raise or lower a Fed funds rate. All countries would fight to have a designated reserve currency. Unpeg the dollar from gold and create a petrodollar by signing an agreement with an oil producing country, which could make resources more valuable in the game, and flood your currency to other central banks, causing inflation in those countries who in turn can buy your treasuries (debt) to offset the flux of incoming currency. This could be a huge part of the end game, albeit, it might require too much of a learning curve for some players on how the economy works.