I'll start, but when it gets hard I'm going to toss it that way.
I think it is possible to design a city, and in most instances you are not going to start from scratch. People are already living in cities, and cities are growing. I think the cities that are most successful are the ones that think ahead to where they want to be in 20, 30, 40, years. Features like waste treatment plants and roads are necessary infrastructure for us to be able to live in places like cities. Successful urban planning has to incorporate the needs for these things.
I am not an urban planner, but when you are considering it, I think you might have a plan that focuses a bit more on, say, open spaces, green spaces, and on how to connect those. Maybe if there's an unsightly area, you'd think about having a bit of a green space around it to try to mask it or to provide a break between it and where the people are actually living. Those are some of the issues I think city planners would want to think about.
A city like Edmonton is very well thought out. They have conservation corridors. They've put a lot of thought up front into how to create a city that is very liveable but which still provides a conservation outcome and the services that all of us as city dwellers need and want.