What would happen to a vacant lot if it was left alone for 25 years

The lot would most likely be home to a mass of plant and animal life depending on where it is located.

A vacant lot will change and include more plant and animal life as it's left alone. Grasses and weeds would be replaced by bigger plants, shrubs, and small trees as the plant community changes. Animals will also begin to inhabit the area making it change as well.