If our country were to start over with a new constitution (say, if there were an actual Second Civil War†), or the voting public got a taste for radical political reform, then these are the changes that I would advocate for:
eliminate malapportionment and strictly adhere to "one person, one vote"
eliminate partisan gerrymandering, for instance by having a truly independent third party do the redistricting
anything that would encourage more than two parties to form, such as proportional representation