Any song that is so wedded to a particular rock performance of it that that remains the definitive version. Try "Jumpin' Jack Flash". "Sympathy for the Devil" can nearly get to folk as can "Street Fightin' Man".

Personally I do a folked up version of "Substitute".

Or you could go for classical stuff that is not convertible into folk. Maybe Dvorak's "Stabat Mater". Psalms, you may be shocked to hear are surprisingly convertible.

Mid you I have heard a number of classical singers totally murder Barbra Ellen. Why they so often go for that baffles me.