Bernie Sanders is farther left than Warren. Neither are credible alternatives to Clinton, whose own standing has been diminished over the revelations of the donations to the Clinton Global Initiative. She's scandal ridden and has plenty of missteps and faults that can be pointed to over her nearly three decades in the public spotlight. It's not a matter of if she makes any more gaffes (such as claiming she was "dead broke"), but when. She can try and act like a poor standard bearer for progressive values, but the left wing of the Democratic Party will never be comfortable with her. This is displayed through numerous polls showing she has 60% of Democratic support despite half wanting a primary opponent. She's always been seen as detached, calculating, and politically motivated.
I believe it will still take a credible opponent to defeat her, considering she is a woman and the fact that perception is reality in politics. Now, I'm not one to judge candidates based on gender or other characteristics which have nothing to do with policy, but there is an undeniable portion of the electorate that sees her for (quite literally) for what she is: a woman; and no doubt, these people will try and make history as they did, albeit poorly, in 2008.
Out of the current field of GOP presidential hopefuls, I am pining for Jeb Bush. He can reach out to the Hispanic community as much as, or perhaps even better, than his brother did in 2004, and he is more moderate than his other challengers (both in perception and reality), so there's some advantages. In my opinion, any candidate who ran in 2012 plus Ted Cruz and Rand Paul don't have a chance. I don't believe the Republican Party is ready to nominate a Libertarian (I definitely don't support Paul), and Ted Cruz has simply staked out positions that are too conservative. Cruz hasn't also been the greatest of companions in the Senate either. Even though I do believe he's a smart guy, the positions he's taken to occupy a niche and to be able to establish himself has only cooped him up, and I can't see him even winning one primary state. As for Marco Rubio, he's had only Senate experience and he's flip flopped on immigration. Even though he is Cuban himself, the positions he's taken don't really line up with the majority views of Hispanics as well as Jeb's does. As for Chris Christie, I think he is politically dead. Perhaps in another field where there weren't as many suitable competitors, he might have a chance, but the numerous credit downgrades in New Jersey and the (unfair) media scrutiny on Bridgegate have only hurt him.