Currently the game only includes GDP, but NNI is a far more accurate definition of national accounting and would help tremendously:

a) The game measures gross investment, not net investment, so depreciation information isn't available,
b) The game has no measure of net foreign factor income which means you can't evaluate your country's return on investment in foreign countries versus other countries' return on investment in yours, and
c) The game doesn't measure the impact of taxes which withdraw wealth from the economy.

As it is, countries that have a lot of depreciation, foreign investment, and high taxes look like they have bigger economies than they really do. NNI would give countries with durable investment, strong capital accounts, and efficient budgets the credit they deserve.

I say this especially in light of countries that have strong currencies which gain a strong advantage in foreign investment. Yes, strong currencies give foreign countries an advantage to sell products in your country, but they give your country a strong advantage to buy production in foreign countries to sell products with. Strong currencies also make it difficult for foreign countries to buy debt in your country while your country can buy debt cheaply abroad.

As it is, the game seems highly biased towards consumption-based economies and against investment-based economies. I say this especially in light of migration where domestic citizens might live abroad, but still claim citizenry at home. Their wealth should still count towards their homeland where their accounts are located even if the physical means of production are located abroad. It doesn't matter where production exists. What matters is who owns it.

On the side, migration should be adjusted for this as well. There's a difference between residents and citizens.