Technically FDR ran on a fairly conservative platform before getting into the presidency and then realized he needed a much more expansive, left-wing solution. So maybe we should look for someone who runs as a semi-conservative but is actually left-wing? I don't know, the exact analogy doesn't seem to fit modern politics that well.
But I do agree that the Democratic bench is very lightweight at this point. Maybe Kamala Harris? I'm not sure.
That's why I don't think people should always immediately write off politicians that at first appear to be "not liberal enough," or too wealthy, or whatever. They
could surprise you. It's not always so black and white.
However, I feel it's worth noting here that FDR/Democrats had massive majorities in Congress for the first 6 years
(and still significant majorities up until 1942), which allowed him to pass so many favorable policies. Democrats also had huge support at the state level, so that helped too. It's not enough to just want another FDR-like president. We need a governing
coalition large enough to implement the policies we are fighting for. That's a much more difficult goal.