We too like the shape, but hackberries do need regular pruning, so it's good to being tackling that when the tree isn't too big. You can prune smaller branches (less than 1 inch in diameter) year round, but wait until late fall or winter to trim larger branches.  It looks as though your tree has two leaders growing at the top, and you'll want to remove one of them.  You can also trim back branches.  Attached is an article on pruning hackberry trees and another from the U of Mn on pruning trees in general.  It is detailed and explains how to make your cuts. 
 https://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs
 s://www.gardenguides.com/75937-prune-common-hackberry.html#google_vignette