I'm all for liberating folk from its sepia vignette, which is why I questioned the OP about what she/he meant by 'motifs'. Almost every folk or folk rock album of the 70s and 80s had a sleeve with the visual equivalent of a hey-nonny-nonny lifted from some Victorian novelty arcade or broadsheet.

As for outsider, well Alfred Wallis was certainly that. I'd image Ben Nicholson lauded him for a 'folk' survival of pre-Renaissance perspective which happened to coincide with modernism and a romantic revival and bridge the two. Didn't stop Wallis dying in a pauper's grave while his paintings are bought by the exceedingly wealthy. Now That's What I Call A Folk Process (Volume 7).