I think the key thing is to frame the new city as a city, and not just an extension. As part of that you'd need to keep enough space to let business pop up, etc

I think the key thing is to frame the new city as a city, and not just an extension. As part of that you'd need to keep enough space to let business pop up, etc. I think part of the problem extensions run into is that they don't want to be a city.