Sure, there might be a few -- a very small few -- bona-fide brown zones here and there that are virtually depopulated and where one can more or less from scratch. But by and large, and realistically speaking, the real question is how to redesign and rethinking existing cities. Which conceptually speaking is a much harder question.
The other elephant in the room is the inevitability of massive forced dislocations of existing populations in these "failed" urban areas (or so they will be declared) -- perhaps on a scale never seen before (that is, outside of modern China or the early decades of the Soviet Union). And BTW, there's no getting around the term "forced" here -- no matter how you try to incentivize it, some people just aren't gonna want to uproot themselves (and destroy the organic connections they have made with others in their "failed" or "poorly optimized" communities) for the sake your grand vision, slide decks and TED talks.
It's either that -- or build out in the jungle somewhere. But of course (with the earnest suport of a certain very famous YC board member) we've been down that route already:
there's almost no room left on the planet for building
"new" cities of any significant scale (meaning of
course not just the meticulously designed city core;
but the whole meta-city that will grow around it --
with suburbs, transportation links, etc)
Which aren't ringed by (and hence, downwind from) mega-ranches? Or correctional facilities? Which have access to adequate water resources (already heavily constrained in many of these areas) to support massive human settlements? And close enough to lakes, parks, and other attractions so that people would actually... want to live there?
Not so many.
And the genuinely fallow areas which do exist should probably be protected as such, and/or restored further to their pre-cultivation state.
(And the amount of farmland we're talking about, both for new cities and for sprawl, is small as a fraction of total farmland.)
This idea of building a brand new city is just stupid.
I suppose if Silicon Valley types like YCombinator who invest the money, and Google (Larry Page regularly talks about Google building a city) who is a large industry were to go forth on one of these model city type projects, the initial draw would be those exact things. If Google wanted a model city to work, Google should ensure that Google is the first business to move a large portion of it's workforce there with major incentives for people to do so. People serving that initial seed workforce other services would follow.
A new city would be founded much the way they did in the old days: Company towns.
Aside from the environmental issues, and the fact that not too many people would be crazy about living there -- what about the generations-old farm families who just... don't want to sell? At any price?
Which is what I meant about there being few places of significant scale (aside from protected wilderness areas that ought to remain as such) that are genuinely "empty."
Consider Astana (Kazakhstan), Ordos (China), Shenzhen (China).
Not to mention the... ironic outcomes of some of these spectacular initiatives: