You will always be well served by selecting from native species - they are well-suited to locale and can be resistant to many disease and insect issues (but not always!). Plus, they encourage birds and other wildlife. This is not to say you must restrict yourself to native species, just that they make a great place to start.

The attached link is pretty informative as to size and specific attributes but you can always Google under botanical name to locate images and more details.

Here is a link that might be useful: Illinois native trees

Red Maple (October glory is my favorite for fall color)or Red oak would fit in nicely in a naturalized setting. If you want something smaller, Dogwoods, Paperbark Maple, Parrotia all have good fall colors.

Sassafras, Black Gum, Sugar Maple, Shumards Oak, Red Maple, Flowering Dogwood, and American Smoke Tree all have excellent fall color. I agree with gardengal48 when she says," natives are a great place to start". You will not get any more natural looking than natives. They are also easy to care for most thrive on neglect.

buy NOTHING that claims to be FAST GROWING...

and my best suggestion.. is to ID what is already there.. and work from there.. diversifying.. and adding things that will compliment what you have...

as of stating your facts. frankly.. your only thought is to fill a hole... and one might suggest.. that lacks foresight ...

e.g. if they are all redbuds.. and you plant a redbud.. well.. you got one more redbud...

if the whole fall color scheme is red.. and you add more red.. you got red.. instead.. lets say.. a birch that turns yellow..

if they are all deciduous.. why not a conifer ....

what you have .. is much more than a hole ... you have an opportunity to create something ...

ken

Hackberries have yellow fall color, interesting bark, and will reseed themselves. Sugar maples, as well as red and white oaks do well on well drained sites. Some sugar maples turn yellow/red/orange, others just yellow. Some oaks have beautiful colors, others just brown leaves that hang on. But even that is interesting in the winter when everything else is bare. Mix in some white pine for added winter color. Just north of here in the Wisconsin Dells area you often see white pine growing in the deciduous forest. I don't recommend red, Austrian, or Scotch pine. Too many problems. White spruce is another possibility. Avoid ash for now with the EAB problem. Aspen groves have beautiful yellow fall color and grow fast and will spread in a natural area. American basswood is another native tree with yellow fall color.

Nuisance-free and fast growing are generally mutually exclusive.