There's a huge non-"mainstream media" on both the right and left already (the term "mainstream media", at least its use as a pejorative, was popularized by the large institutional right-wing media, there's also a substantial non-traditional left-wing media [which is where most of the stories that generate anger on the left with the traditional "mainstream media" on the left are transmitted].)
The anger isn't a sign of an open opportunity for non-traditional media on the left and right, its the product of the success of existing non-traditional media in transmitting ideas within the left and right.
A lot of the people I talked to expressed disgust with vox, vice, slate, and salon as becoming establishment not just the washington post.