Just about all my faves are "out of the mainstream", to one degree or another.
Welshman Dave Edmunds recently announced his retirement (he is now 75 years old), and was for many years my favorite Rock ’n’ Roll guitarist, and producer (Rockpile---his band with Nick Lowe, The Everly Brothers, The Fabulous Thunderbirds, Carlene Carter, K.D. Lang, The Flamin’ Groovies, The Stray Cats, Jimmie Vaughan, Mason Ruffner, Foghat, many others).
Thank God there is a guitarist/singer/bandleader (for Emmylou Harris)/producer who has taken Edmunds’ place: the great Buddy Miller. Whether solo, with his wife Julie, Emmylou, or other collaborators (Jim Lauderdale, for one), all fantastic music, imo amongst the very best being made today. Listen to Buddy’s recording of Tom T. Hall’s song "That’s How I got To Memphis" to hear real white soul!
Is John Hiatt mainstream? How 'bout Rodney Crowell? Emmylou Harris? Patty Loveless? Marty Stuart? Richard Thompson? I KNOW Iris Dement isn't, and she's at the absolute top of my list, a list very long due to the fact that a LOT of great music is being created right now.