A certain trend that has come up is planting trees and small gardens in the city area. Some places have gone 10 steps farther and began planting on rooftops. This is wonderful. It doesn't exactly counter the effects of all that traffic but it doesn't hurt to try. I haven't actually seen what they're planting though but at least if they are invasive, they're not in a good spot to spread.
One tree that I know they've been planting is the Bradford Pear. Basically a "Pear" that only produces flowers. But here's the problem, farther south it actually makes fruit, (a small green/yellow crabapple like thing). And the trouble is it's invasive. So maybe the city is the perfect place for it. Yet another problem is it's a softwood tree. Literally the trees fall apart under harsh wind. So they're not the best thing to plant where people will be parking their cars. Another issue with the Bradford Pear is it's useless. The only thing it's there to do is flower. It is a nectar source but the reason why is usually doesn't produce fruit is because it flowers to early for anything to pollinate it. Thankfully the cities around me have stopped planting this tree.
I have no idea what they're planting now but I'm trying to think of what in the world be best. See I can't get around the car thing. I want to say some sort of Hardwood Oak or native Maple would be best. Birch, and Redbud would be nice. Mostly because they don't fruit a whole lot but are used as host plants bye loads of insects. A Willow tree might also be good, (that is to say a true willow, Salix,) but those are hard to fit anywhere. So I'll leave the decision making of trees to the people who know best for city life.
Cities are so void of biodiversity in general that I think almost any sort of vegetation is a godsend. Particularly if they're supporting hundreds of species of butterflies, and birds, even if it's just a season.