To put it non-elegantly, two lying old biddies does not a balanced ticket make. If Hillary! somehow survives, physically and legally, to obtain the Dem nomination, she has to pick a Marxist male of color. Julian Castro is the obvious choice for them.
Some caller on C-SPAN this morning, a Bernie supporter in Florida, was making the case that Bernie would be the best person for Hillary to pick because of the enthusiasm it would generate for the ticket.
Seems very unlikely--Bernie is too white and too old.