I feel like MacArthur should turn America into a Statship Trooper or Roman Republic type of government especially after winning the civil war. That seems like the society he would build. If he lives America goes the Starship Trooper route if he gets assassinated America goes the way of the Roman Republic. He seems like he would enforce a meritocracy with some democratic elements. Things like religious freedom, freedom of speech, racial equality, and social or economic mobility are respected but if you step out of line they punish you extremely. Your free to speak your mind but if you get violent in anyway they have no problem shooting you in the street. Also if you words bring suspicion of possible civil disobedience they will allow you to speak but secretly being spying on you looking for any illegal activity relating to possible partisan or treason like activities. Things like corporal punishment, flogging, and forced labor being common punishments. MacArthur would probably try to set this up in a constitutional and balanced system to try to prevent abuse of it or seizure of complete dictatorial power by one person. The government is also probably much more centralized.