I think a formidable third party is totally viable. Especially now. Guys like Bernie, who could get out of the gate with a healthy base, are just too chicken shit to strike out and give it a shot. I don't think they'd have a problem fundraising and getting a healthy war chest together either. It would just take some time to build up, and be a pretty big gamble career wise, but you're never going to get anything without somebody like Bernie who has that built in appeal, splintering away from the Dems.
Fancy yourself a "radical progressive", or "revolutionary" who wants to shake up the system? Act like one.