Other possibility to accommodate more people is Seasteading.
The basic idea is the same: to be self-sufficient and to aim for the skies to avoid space constraints. But what this vision adds is that one can be independent from an old state and live with new rules and lower taxes.
The “floating” cities would then be built on off-shore platforms (similar to the oil ones). The city would then include hospital facilities, schools, universities, utilities, … and even a legislative body!
Here is why it ought to exist floating cities, according to their own words:
Why The World Needs Seasteading
The vision of seasteading is an urgent one. We can already see that existing political systems are straining to cope with the realities of the 21st century. We need to create the next generation of governance: banking systems to better handle the inevitable financial crises, medical regulations that protect people without retarding innovation, and democracies that ensure our representatives truly represent us.
Seasteaders believe that government shouldn’t be like the cell phone carrier industry, with few choices and high customer-lock-in. Instead, we envision a vibrant startup sector for government, with many small groups experimenting with innovative ideas as they compete to serve their citizens’ needs better.
Currently, it is very difficult to experiment with alternative social systems on a small scale; countries are so enormous that it is hard for an individual to make much difference. The world needs a place where those who wish to experiment with building new societies can go to test out their ideas. All land is already claimed — which makes the oceans humanity’s next frontier.