Never happen. No way could we get this lucky.
Thought experiment: What sort of calamity would have to befall the Democrats to make Joe “Chains, Y’all” Biden a serious contender for the nomination in 2016? If Obama loses to Romney this time, Biden’s damaged goods and destined for retirement. No sane liberal would want to roll the dice on the weak link in this year’s Democratic ticket against President Mitt when the strong link couldn’t get it done. If Obama wins, then Biden’s kinda viable in 2016 but still facing looooong odds. He’ll be almost 74 on election day that year, and he’ll almost surely be facing a strong Republican nominee given how deep the GOP’s bench is right now. When Bush 41 followed Reagan’s two terms with one of his own in 1988, he succeeded a popular, successful president and faced an exceptionally weak opponent in Dukakis. Unless O’s hypothetical second term turns out to be vastly more productive than anyone expects, Biden will have neither of those advantages.