I agree, lots of Gordon Bok, and also Bob Dylan's "Lay Down Your Weary Tune" which is the one Bob Dylan song that should be sung by traditional singers who do not do Bob Dylan. Also, several of Eric Bogle's not only should be but are well on the way, the (probably obvious) "The Band Played Waltzing Matilda" and "No Man's Land" but also some slightly less heard ones like "Leaving the Land." John Prine's "Paradise" and "Angel from Montgomery." Bill Staines' "River" and "Roseville Fair." John Denver's "Country Roads" is virtually there too, as is Steve Goodman's "City of New Orleans." Like Jean/kytrad's work, John Connolly's "Fiddler's Green" is often presented as folk, and Larry Kaplan's "Old Zeb" is too.
And, of course, the one Grateful Dead song that I think will be sung as "traditional" a hundred years from now, "Ripple."