The other possibility is him as FDR's running mate in '44. FDR told his advisers he could accept Truman or Douglas. Had Truman turned FDR down, Douglas could have been the nominee (though he'd have faced substantial southern opposition — possible the eventual choice would still have been something else).
Still, that's probably the best bet. Douglas as FDR's VP, becomes president in April '45 once FDR dies. As an ardent New Deal liberal he might well pursue less hawkish policies vis-a-vis the Soviet Union, but will likely suffer domestic problems as the Cold War and Red Scare set in.
And with substantial opposition from southerners, he might have a harder time winning renomination than Truman and a harder time winning reelection overall, especially with a stronger Dixiecrat revolt / third-party candidacy.