Create Islands that very rich people can purchase (these would be chunked onto separate servers, including the possibility of hosting your own city). Users would still authenticate through the central game server.
Make a world builder that allows new islands and cities to be created. New Cities would be planned in the world builder, but then you would need to employ construction crews to build the roads etc.
Infrastructure such as electricity, gas, water, brown and grey water sewage would also need to be installed. As building are added this infrastructure may need to be upgraded. Street lighting, traffic lights, CCTV cameras for Crime Prevention etc needs cables to a central computer that operates it.
The city owner (mayor, king, dictator) could zone land, so other players could buy land and construct buildings. The city owner could specify size of footprint and max height of buildings, and type of buildings.
New Islands and cities would have to create their own infrastructure, such as transport links (air, sea, road).
In order to make a new place thrive the city owner would need to attract a population, and then facilite the needs of that population.
Create Passports and Immigration Rules
City combat mechanic