It's a half baked idea of mine. I'm not actually in favor of a third party presidental candidate. Just Bernie starting one as a power play. Forcing the Democrats to embrace class based economic messaging and ideology as the cornerstone of the party. Bernie is 83, it's best for him to leverage his celebrity and cross over appeal to influence change and elevate the profiles of the next generation of like minded candidates.
The third party would be an, ideally, toothless way to enable that change by endorsing a candidate in the dem primary. And have a long, public and transparent process for that endorsement.