Okay you can change anything you want about America(for the better)but you have to keep America OTL in terms of territory.
A couple of my ideas are
Better Constitution(bill must pertain to single issue, no three-fifths compromise, Judicial Review and power of enforcement to supreme court)
Samuel Morey(A successful internal combustion engine in the 1830's who knows what could happen)
Peaceful end to slavery followed by successful civil rights for all by Progressive Era