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.