While I agree with your statement that independent city states are the future of governance in general, we are always going to require a broad coalition government. This is because actions taken in one place frequently spill over to another, and there needs to be a mechanism for redress in these circumstances. For example, a factory upriver dumping waste legally according to its municipal regulations affects everyone downriver of it. Additionally, there needs to be a check in place against tyranny by the majority at the local level.
I think a best case scenario would be popular-vote based local (metropolitan area) democracies, with a "world court" system that protects rights and arbitrates disputes between governments.
There can be no independent abstract "world court" because there is no military to back it. The closest we have is probably the united nations security council, which is composed of US, Russia, China, France, UK + 10 others that vary over time. These actors have varying interests which keep each other in check to some degree.
A world court doesn't require a military in a highly interdependent world. Economic sanctions are sufficient. A city state is going to require food and goods from other city states. If you go against the ruling of the world court, other city states refuse to provide those goods, and you are going to fall into line fairly quickly.
Another benefit of smaller independent city states is that it is easier to check their aggression. If raising a large army requires broad cooperation among city states, that greatly reduces the likelihood that such a force will come into being. This is because of the difficulty in coordination and the fact that multiple city states are unlikely to share the exact same interests, reducing their impetus to unite.
I think a best case scenario would be popular-vote based local (metropolitan area) democracies, with a "world court" system that protects rights and arbitrates disputes between governments.