Every day proves Thaddeus Stevens right: there should've been big Nuremberg-style public trials of leading Confederates, accompanied by seizures of the assets of all pro-Confederate large landowners and businessmen, which should then have been distributed to freed slaves as compensation.
The Confederate states should not have been readmitted to the union. They should have ceased to exist. They forfeited their sovereignty by rebelling. The territory should have devolved to the federal government and been administered by it indefinitely.
Maybe in a few decades, after the slaveholding class was long dead, you could have formed some NEW states out of that region. You know, Grantland, Shermania, that kind of thing.