Bill Kristol is an idiot and he doesn't have his finger on the pulse of the Democratic Party. He is also a concern troll and wants to argue that OMG Hillary Clinton's campaign is in trouble.
I think the only potential major candidate is Biden. Maybe it's about a 30% chance he jumps in, who knows? The Times just posted an article about the Biden's camp putting out feelers to donors and Maureen Dowd reported that Biden is discussing the possibility.
Hillary Clinton's campaign is facing some challenges given the candidates ethical track record and the fact that she is barely to the right of Bernie Sanders, the base of the Democratic Party wants someone who is unapologetic about their progressive values. In 2008, the Democratic base went with an anti-war progressive who could win. While Barack Obama has done most of what the liberals have wanted in terms of economic policy, same-sex marriage is not legal at the federal level, he hasn't pushed for gun control, and Obamacare is government regulated health care, it technically isn't government run health care although medicaid has been expanded. Sanders is unapologetic in his liberal beliefs and would expand Obama's liberal policies, that's what the base is looking for and that's why Hillary is in trouble, she'd simply be a third Obama term.