"Building tall buildings underground, instead of above."
First, building up is much less expensive than building down. Second, having been in a bunker with false light systems... you get to miss even the small amount of natural light that you get in the center of current skyscrapers.
"Requiring high altitudes for all planes, military or civilian, and producing auto-shoot auto-aim turrets around the ciy with no warning shots."
Requiring that the airport be a significant distance from any city or the planes are shot down before they get to "approved height". How far to you want to drive to the airport? It would also be putting interesting weapons around everywhere for people to play with... just imagine the first hardhack tagged story around that.
"Include parashoots as standard emergency materials for skyscrapers?"
Wind around skyscrapers is very hazardous, look into wind tunnel effects that are reported around skyscrapers. Zip lines would be safer but impractical in other ways.