What would be really interesting to me would be a song chain showing variants of
existing folk songs and their metamorphoses through the ages.

Example: The Ballad of Robert Kidd (Pirate)
Sam Hall.
What Wondrous Love is This (shape note hymn)

The stanzaic forms are similar indicating influences that changed the variants.

Variations for example of the Binorie Ballad, The Twa Sisters, The Wind and the Rain, etc.

Or The Three Ravens, Billy MacGee Macgaw, and possibly the melodic content for Johnny I Hardly Knew Ya'.

Every folk song in tradition has an antecedent either in music or in story content.