Biden would move up to the first slot.
He'd have to appoint a Veep.
Hillary!™ is the most logical choice.
(McCain-Feingold would prevent an effective counter campaign)
Biden will step down for health reasons two years into his term.
Hillary!™ gets the last half of his term, and can still get two full terms of her own.
History text books will refer to this time as 'The Glorious Hillary!™ Decade'...