I've just read a great article, "The Biden Agenda: Reckoning with Ukraine and Iraq, and keeping an eye on 2016." by Evan Osnos in the July 28, 2014 New Yorker.
It suggests to me that failing Warren or someone like her, Joe Biden might be a 2016 primary advocate for the 99% and an alternative to the hawkishness of Hillary Clinton.
Some pertinent passages from the New Yorker article:
Since entering the Administration, Biden has been a strident voice of skepticism about the use of American force. At times, that put him on the opposite side of debates from others in the Administration, including Hillary Clinton and Leon Panetta, Obama’s first C.I.A. director. Biden opposed intervention in Libya (as did Defense Secretary Robert Gates), arguing that the fall of Muammar Qaddafi would result in chaos; Biden warned the President against the raid that killed Osama bin Laden. If it failed, Biden said later, Obama “would’ve been a one-term President.” Though Obama heeded Biden’s advice only sometimes, the two men adhered to a restrained foreign policy that “avoids errors,” as Obama put it to reporters in April. Asked to articulate an “Obama doctrine,” the President said, “You hit singles, you hit doubles; every once in a while we may be able to hit a home run.”
When he was in the Senate, Biden was a centrist Democrat who called on his party, at times, to back diplomacy with force. Though he voted against the Gulf War, in 1991, he advocated NATO air strikes in the Balkans in 1993 to stop the Serbian slaughter of Bosnians. In the run-up to the war in Iraq, in 2002, he pushed a resolution that would have allowed Bush to remove weapons of mass destruction in Iraq but not to remove Saddam Hussein. The resolution failed, and Biden voted for the war, a decision he regrets.
In the Administration, [Biden and Hillary Clinton] differed sharply on the use of American force; she favored a surge in Afghanistan, a mission to depose Qaddafi, and the bin Laden raid, and he opposed all three.
Writing for The Atlantic in May, Peter Beinart took stock of Clinton’s momentum and concluded, “Joe Biden’s prospective Presidential candidacy is in danger of becoming a joke.” Beinart mourned that development, arguing that the contrast between Biden and Clinton on foreign policy could spark a debate “about America’s role in the world.”
Biden had recently taken to making comments that might position him as a progressive alternative, à la Elizabeth Warren, the Massachusetts senator. “I have a basic disagreement,” he told me, “with the underlying rationale that began in the Clinton Administration about the concentration of economic wealth.” As Hillary Clinton fended off further questions about her income, Biden told an audience in Washington that he was wearing a “mildly expensive suit,” despite not owning “a single stock or bond” or having a savings account. (In fact, his family has securities in his wife’s name, and a savings account. Tax returns show that they are heavily mortgaged.) Jon Stewart declared it “a good old fashioned Poor-Off.”Even if Clinton wins the nomination, a decent competitor might force her to moderate her aggressive foreign policy tendencies.