Depends on how the war ends. Do the Chinese not get involved and MacArthur conquers North Korea, or do the Chinese get involved and we get pretty much OTL but with MacArthur remaining in command? That will affect his image and determine what he does post-war. He was in his seventies by the time the war started in 1950. If it ends in 1951, then he'll probably retire. The war ending in 1951 also means that Truman's popularity won't take as big of a hit. Maybe Truman runs again in 1952? I just don't see MacArthur getting very far if he runs for public office. Like Patton, he could never learn to keep his mouth shut when he should have. A couple of gaffes (and he would make them) would end any political aspirations. Most likely he goes into private life and makes the occasional political statements........much like OTL.
Also, Truman wasn't looking for a reason to fire MacArthur. MacArthur provided the reason. To quote Truman: "I fired him because he wouldn’t respect the authority of the President…I didn’t fire him because he was a dumb son of a bitch, although he was, but that’s not against the laws for generals. If it was, half to three-quarters of them would be in jail.”
Truman knew that MacArthur was very popular with the American people and that Truman would take a massive hit if he relieved him. So if MacArthur can keep his mouth shut and respect Truman's decisions (to say nothing of his position as Commander-in-Chief), then I have no doubt that he would remain in command for the duration of the war.