Martin Heinrich (D-NM) (Senator Dreamboat) has taken up the mantle of attacking the Dakota Access Pipeline. He could be a good candidate. Pro-environment and anti-mass surveillance, but he doesn't have a widely publicized or controversial background to my knowledge (that's what holds Sanders and Warren back). This could be a start of Heinrich raising his profile for a 2020 run. He's pretty young (he'll be 49 in 2020), he's had two terms in the House (2009-2013) and will have been in the Senate for about 8 years. He also doesn't have the stigma of being a coastal liberal.
http://krwg.org/post/heinrich-calls-president-reroute-dakota-access-pipeline