Several people have written cogent comments on my posts on science fictional cities (The city in science fiction and Charter cities). I am intrigued by the concept of Charter Cities, or maybe Challenge Cities: the equivalent of "stadtluft macht frei" (city air makes you free) of the European Middle Ages, where you could escape bound status by fleeing to a city.
Of course, in that era, rulers had to do that to make sure cities were inhabited at all, because their death rates were so much higher than that of the surrounding countryside. If there wasn't in-migration, their populations would have dropped.
So I really am thinking about the possibility of being able to escape to a jurisdiction with another economic system without having to migrate to some other continent. It's the ultimate in diversity. But, of course, cities are never purely economic entitities. And, despite what some extreme libertarians would like to think, you can't build a public life on purely economic relations. Politics--power--always raises its head. Coalitions form. People regulate each other. Each group tries to zone for the lifestyle it considers ideal. A successful city without significant political power seems unlikely.
And this is all to ignore the notion of the city as aesthetic object, which is an significant omission. We have forgotten, since cities were always objects of power and of wealth, but are now seldom objects of beauty, save in fugitive and chance ways. Beauty involves making a set of consistent choices, and that implies that other choices are not made. There are many diverse beautiful objects, but each beautiful object is jealously non-diverse. Just as, in the pre-modern era, tyrannies tended to permit women more freedom than democracies, so beauty depends, at least in part, in a constriction of freedom.
Of course, that might imply that there have been, somewhere, unfree cities that were beautiful. I can't think of any. Kim Jong-il's Pyongyang? Ceaucescu's Bucharest? Stalin's Moscow? Not promising notions. Still, I think there is something to a constriction that requires a theme-and-variations approach to structure. But it can't be imposed by some external force. It may be a choice that can no longer be made.
Or is it just the need to deal with cars? If so, that's another choice we're stuck with. No one's building any cities that can't accommodate cars. That would be like building a beautiful house without indoor plumbing. And I say this as someone who seldom drives.
So, cities of wealth, cities of power, cities of culture. A lot to play with here.