There is no incentive for countries with high GDP value unless citizens those countries get to freely travel to, migrate to, own properties in participate in politics in, and do business in countries with lower GDP.

That is what countries (like Texas) that joined the United States had to concede, albeit reluctantly. Texas had to adopt US government rules along with currency.

This is why the European union is having issues, their "federal" government is at odds "state" government.

We need to have everybody on board in term of having similat governing rules and culture first. Only under such conditions will a union be entertained and a global currency even be accepted.

The only way that is going to happen is lowering border and travel restrictions and allowing people to free do what they want with out too much goverment restrictions. In this wayb culture and government will become slightly more homogenized.

That aint happening in the current isolationist political and social climate. Terrorism and a ten year recession has generated enough fear in the world that we have a record number of countries and states becoming more isolationistic and attempting or considering leaving their federation.

Let's wait 20 to 50 years and see how it goes. Though I'm skeptical China will ever consider it. Oligarchies tend to want to retain their autonomy, unless they are willingly given control of the federation by US, Europeb or Russia...which, for understandable reasons, will never happen, lol.

" is the euro. It is the currency of the world’s second-largest economy". stopped right there. the euro is the currency of the world's largest economy.
but they don't have the military threat which the US possesses.

the monied folk will do what they want.