Rather than "songs that should have been folk songs", it's often easier just to see who's a successful singer-songwriter. If they are, chances are that many of their songs are going to become folk songs given time, because most singer-songwriters transcend the standard categories. Mark Knopfler is one; others are Suzanne Vega, Ani DiFranco, Tracy Chapman, Joan Armatrading, Chris Rea, John Prine, Steve Earl, Bob Seger, Billy Joel, etc, etc, etc.

Graham.

PS. If you were considering it, don't buy MK's "Shangri-La". Very disappointing album.

PPS. On the MK front, "Money for Nothing" is very possible as a solo acoustic number on a single guitar. Try that one at your local folk club! :-)