What if the timings of the deaths of FDR and Stalin were reversed? So now Roosevelt survives until March of 1953, while Stalin dies in April 1945. Say Roosevelt stopped smoking at some point and took the occasional bit of time to relax, giving him an extra few years, while Stalin fell to the same mysterious mix of natural causes and possible poisoning as in OTL (not too unrealistic: he suffered a major heart attack just a few months later, in October 1945).
This means that Roosevelt manages to see out the entirety of his final Presidential term, and thus the rest of WWII. Particularly with Stalin out of the picture, Roosevelt would exert a huge degree of influence on the post-war order: probably much more than the relatively inexperienced Truman who was thrust in to replace him. I would assume he does not run again in 1948, having finished out the war and still not exactly being a healthy guy. What's less clear is who replaces him on the Democratic ticket in 1948, or how politics will proceed from there.
The situation in Russia is even less clear. Who can succeed Stalin at this point? Certainly not Kruschev, who is the Ukraine. The most likely candidates are probably Beria, Molotov, or Malenkov, although in the chaos there are plenty move possibilities. Depending on which of them gets to power the start of the cold war could go drastically differently, or be avoided entirely.
What would the biggest effects be of Roosevelt's continued presence on the world stage and Stalin's absence? What policies were championed personally by those two rather than being fairly inevitable regardless of who was in charge?