I always found this view of MacArthur leading a miltary junta pretty ridiculous unless there were some big butterflies involved. An example of this would be Jello_Biafra's great TL's
Reds! A Revolutionary Timeline and
A Red Dawn: American Revolution and Rebirth. The only reason he is leading a junta is due to the fact that he is leading a Taiwan-like nation with a communist US next door.
MacArthur's America would most likely be more nationalist but not on a Nazi Germany level. Being the general that he was there will support a larger increase of the US military than OTL. As for the Red Scare it would go on a bit longer due to MacArthur once telling Kennedy, when he was president, that he should be fighting the communists within the US before fighting them in the jungle. The Red Scare will end however because if you look at OTL although the Ike administration didn't like McCarthy it was McCarthy's fellow Senators who brought him down with censurship.
Diplomacy-wise MacArthur is going to be more Asian focused rather than Europe in the Cold War. Under him we might see SEATO survive like NATO and actually help fight communist expansion in the region. MacArthur will be do some saber-shaking with the Warsaw Pact and with Mao so a Sino-Soviet split might be out of the question here. The space race is going to be more militarized in the future. MacArthur will probably only complete his first term if he survives. He could be reelected but he will die earlier than OTL because he was an older man and the strains of the presidency will end up destroying his health.
A MacArthur presidency TL has always been an idea going on in the back of my head so I look forward to seeing the results.