The biggest gap in this discussion, thus far -- the elephant in the room, as it were: 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).

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:

  Honduras Shrugged
  http://www.economist.com/node/21541391