Honestly, after Japan surrenders, I expect FDR to resign. Wanting to see the end of the war is the only reason he kept going. I don't see him wanting to live out the next 3 years of the Presidency.
If he is healthy though, he might serve as the first General Secretary of the United Nations. Although there are many reasons why an American would never get elected as Gen Sec today, they don't really apply to FDR at this time. With FDR at the head, there'd likely be a very different world than there now is given his ideas and his prestige. The US would have a very different relationship with the UN and the international system in general.
The big question is what he would have done regarding the Soviet Union in Eastern Europe. I don't think there will be a lot of consensus on what FDR would do.