Bad idea. e.g. the Russian and Chinese governments would submit block votes in favour of whichever candidate would most trash the U.S. military and U.S. economic independence. Pollution wouldn’t come into it.
So instead of capitalism be overthrowing democracy in each country in its own particular ways, now it would be able to do it on a global scale.
This brings back memories from Obama's first campaign. My parents joked that with the kind of influence the US has over us, every country should have a say in who leads them!
"Global Voting" is a poor solution to that problem, but more importantly, it is redundant. The Internet ensures that people in other countries can exert some influence over US voters. Even more importantly, migrant communities can exercise some amount of influence depending on their overall relationship with their home countries.
It's still a fun idea to think about.