My first pick for ash replacement is a seedless thornless locust, 'Skyline' and 'Moraine' [Gleditsia triacanthos inermis 'Moraine'] are my favorite picks. Compound leaves, yellow fall color, upright and vase shapes, I do not know of an insect problem that bothers locust.
For size of tree and great shape as a tree I love littleleaf Linden- Tilia cordata 'Greenspire' has a green leaf in the summer, yellow fall color. The shape of the tree almost looks like it has been pruned, but this is the natural form. Spring surprise is the lemon smell to the flower, which is not over-powering.
When pear trees start to break apart from ice and wind, linden are standing strong.
There are a number of beech trees that make a nice lawn tree, the hottest on the market is the Tri-Color Beech - Fagus sylvatica 'Tricolor'. These trees have green in the center, then a pink margin, then a white outer margin all through the season. These are the kind of trees that stop cars on the street.
There are also burgundy leaf beech, fernleaf beech and American beech. Unique, different and collectable.
Another new tree that is great for street tree or yards is columnar sweetgum. First, they are sterile, not sweetgum balls. They are good for tighter spaces, I actually plant more of them in a grove. Beautiful fall color, burgundy, yellow, red and orange. Liquidambar styraciflua 'Slender Silhouette' is the one to ask for. Star shaped leaves.
Elm trees are back, some fifteen varieties are on the market. Watch for names like Ulmus Americana "Jefferson", Princeton, Valley Forge, Frontier, and Homestead. These are resistance strains. Dutch elm disease killed 80% of elm trees in many cities. These are glossy leaf trees with vase shaped tops that will withstand cold, wind and bugs.
If your neighborhood is full with Silver Maple or Pear trees, these are the promising new trees that will bring diversity to your street.
Image (CC BY-SA 2.1 ES) by Luis Fernández García via Wikimedia Commons
Image by Paul Cappiello via PlantPlaces.com