Hillary Clinton: Julian Castro
Bernie Sanders: Chris Murphy
Joe Biden: Angus King
Martin O'Malley: Bill DeBlasio/Gavin Newsom
Jim Webb: Joseph Donelly
Lincoln Chafee: Christine Gregoire
Jeb Bush: Cory Gardner
John Kasich: Joni Ernst
Rand Paul: Jeff Flake
Marco Rubio: Tim Pawlenty
Scott Walker: Pat Toomey
Ben Carson: Ed Gillespie
Mike Huckabee: Pat Toomey
Ted Cruz: Rick Scott
Donald Trump: Jesse Ventura
Please note that some, like De Blasio, would be a Palin-esque "risky pick".
That's one of the few tickets I would actually vote for a non-Trump/Cruz/Paul Republican against.