Not much can be done now due to the after effects of the crash, but in the medium term there is a lot go for. What the downtown clearly needs is employment and residential density. The city needs to push hard to get development started. It should not be worried about high rises several square miles of 5 to 12 storey apartments and offices would transform the city.
The existing strategy of the Biomedical campus and the university district look like good ideas and the faster these trip generators can grow the better.
It is also important that any new office development promotes street life. In recent years in the UK landlords are finding it difficult to let offices on small business parks where there are no facilities and the bigger ones are trying to add residential so they can support more shops and services. It has been noted that many current park occupiers are looking at town centre locations because of the better public transport and the retail and leisure that is there.
The implication being the competitive advantage a city centre offers is it's environment, with other offices, retail and restaurants a short walk from your desk. New office buildings should be hard against the sidewalk with ground floor retail/lesiure units. It really does not matter about anything else as long your streets are filled with active uses and the sidewalks have people on them all day long.
I driven a few US cities downtowns that looked like they had been evacuated, they'd turned themselves into a business park that was a bit denser than the suburban ones.
The key to city centre vitality is a dense residential hinterland. If there were a 100,000 people with a mile of the metro line between downtown and Indian rd, for example then there would be sufficient population to support a diverse retail and leisure scene. This in turn would attract more office occupiers and a virtuous circle.
Getting from here to there is the difficult part.
But if Phoenix resumes it's growth it should not be too hard to get a 200,000 people out 5 million to live in a dense inner core.
The city should concentrate on making the core as attractive as possible to business and residents and have the correct zoning policies so the right form of development occurs.
I've drawn a few quick maps to give you an idea about what is needed. The actual boundaries of these lines don't matter it is just to illustrate broad principles.
Light blue - zone for increased density, multifamily and row homes.
Pale red - employment led mixed use at increased density. New development built to streetwall, active ground floor uses.
Red sites - examples of underused sites that could make good employment buildings.
Orange- sites that are underused, could make good residential apartment blocks.
Purple - University/Bio science.
As the population rises, it would be good to have a few good shopping streets, here I've drawn possible streets that should have stricter rules on including ground floor uses for retail and leisure. Again, the actual streets chosen do not matter, but it is important that retail is not scattered around the area, broken up by car park ramps and blank office lobbies. Not all streets can support ground floor uses so it would be best if certain streets were declared a priority.