Don't Piggy and Kermit already fill the Lucy-Schroeder dynamic, though (sans the piano).
As far as the "If-Rowlf-were-boss-would-Piggy-like-him-instead-of-Kermit" question, I think the answer depends on whether or not Piggy is attracted to Kermit because he's an authority figure, or is there something about the Frog specifically she's in love with?
I would say that she started out liking him because of his position of command, but then grew to love him for him.... which does nothing to answer our main question, because then we ask why wouldn't she do the same with Rowlf?
Aaaah... philosophical debate.... can't... compute....