A Vulture article claims that Lynch is trying to get funding for a film about bluesman Robert Johnson, based on a screenplay that has already been published in book form which has attracted much attention over the years. Most likely this is just bad journalism. Apparently he has praised the script in the past (as has Scorsese) and said he'd love to make the movie if things fall into place, but that hardly means he is actively working on it.
If he is, I'm not too keen on it. I'm not familiar with the subject, but don't like the idea of him doing another movie about a real person. His two previous are classics, of course, but in general I'm weary of biographies of any sort, as they terribly distort the truth. He also shouldn't be filming a script he didn't at least co-write, not at this point. Who knows, though?
Could be something he'll be producing, but not directing. Pretty sure he's also producing a movie that'll be directed by Duwayne Dunham at the moment too.