Polanco has my vote. Baldelli on the other hand may feel it's too late to tinker.
League average OBP is .320. We have exactly two guys with sufficient PA to qualify for a batting title, who are above that. Arraez is injured at present. Nelson Cruz would be a bold strategy at leadoff; I'm not going to suggest we see if it pays off for us.
Of the rest, Polanco is at .318 and hasn't hit with a lot of pop, so he might indeed be a logical candidate. Kepler and Rosario profile similarly, though with a few more home runs.
By a different view, leadoff could be for your best OPS guy (or whatever overall offensive metric you prefer) whose value isn't concentrated in his home runs. Kepler has better OPS than Polanco, but not so many home runs as Rosario. With no ideal candidate present, Kepler is at least in consideration.