Woodrow Wilson (1919)
I have always assume President Marshall makes the necessary compromises to get the treaty passed. Then again Vice Presidents were so out of loop in those days, that while I know he favored progressive reforms and a good five cent cigar, I don't know what he thought about the league.
In 1919, Wilson is still in Paris, trying to negotiate the treaties. Marshall does not have the same views, motivation, or abilities. He probably wouldn't even go to Europe.
Warren G. Harding (1918) Some other Republican wins in 1920, maybe Hoover, and serves two terms. Probably there is no scandal.
But which Republican? An outright Progressive? And Harding was in some ways a very successful President. He and Mellon rode out the 1921 recession, stabilized government finances, cleaned up the tax structure, and set the basis of the 1920s boom.
Neville Chamberlain (1935) Somebody else wants to appease Hitler in 1938.
Again, who? It wouldn't be Churchill. Eden's too young. Halifax is a peer.
Wendell Willkie (1939) Dewey or Taft gets the nomination loses worse than Willkie did.
Are you sure? Willkie was a first-time candidate. Robert Heinlein, then a keen observer of politics, thought that Willkie self-destructed through a lot of first-timer mistakes - and that the third-term issue was a huge handicap for Roosevelt.
Franklin Roosevelt (1941) President Wallace never develops his naive attitudes toward the Soviet Union. He is hampered by his weak political skills, Maybe the presidency is frustrating and he doesn't run in 1933.
ITYM 1948. However - why should the Presidency enlighten Wallace? It didn't enlighten FDR till maybe near the end. Wallace is certain to make different choices from FDR on many issues relating to the war.
And in 1944, the inner circle of the Democrats thought he would be a dangerous drag on the ticket as VP. As President he could easily lose.
Henry Ford (1942) Just as in OTL, Henry II is discharged from the Navy and sent back to Dearborn. Maybe he takes over the company.
The elder Ford's return to the helm in 1943 was disastrous for the company. He nearly ran it into the ground before he was finally forced out. If he's dead, that doesn't happen.
Lyndon Johnson (1968) President Humphrey makes moves toward peace and wins the election.
Unless he is defeated for the nomination by Kennedy, who would declare as soon as LBJ died, not wait for McCarthy to show LBJ's weakness.