Nations are no good in solving global challenges. nation is an artificial identity basingon tribalism. A metropolis identity is good enough for tribal needs while allowing to form much larger planetary scale governance bodies like the parliament of mayors. Therefore the path to stronger earth is strong and independent cities.

Where to have them? All land is owned. We can build new cities in space, in sea or to buy/rent the land from existent countries (much like charter cities but more independent). Sea cities are actively developed now. Charter like cities should have a very strong autonomy, preferably sovereignty, possibly bought with a lot of money. Preferably on sea shore on a border of two countries, with a port and sea airport, water desalination, sewage processing and power facilities.

Who will populate them? Stock holders. People who prefer the city charter over their national idea. Work emigrants.

What form of governance will be there? Charter gives a place for experiment. I personally tend to libertarianism and meritocracy but many flavors are possible.

Who will defend them? Ideally a city joins a local mutual deference pact. It is essentially a civil entity with no strategic depth nor natural resources. It can have an offshore counterstrike capability. A league of cities can have a navy with aircraft carriers, submarines, weapons of mass destruction.