I'd appreciate it if you clarified what you mean by 'non-mainstream'.

I have a book of short stories by Nalo Hopkinson, Skin Folk and they're on the fantasy and horror side, and they are good stories.
She's Jamaican-born and lives in Canada, and is black and queer and a woman (so not the straight-white-man of mainstream). On the other hand, she's won the John W. Campbell award for best new writer (in 1999) and Skin Folk won the World Fantasy Award, and winning awards sorta indicates being embraced by the mainstream, maybe? I don't know.