Hillary! has to run in 2016, because her clock is ticking, and if she doesn't get it now, she will be too old to try again. Everyone else on the Dem side (except perhaps Biden) who would like to run for the nomination can afford, in both the time and money senses of the term, to wait until 2020.
Any Democrat with half a brain can see that the S will HTF sometime in the next five years, whether it be in foreign affairs--any one of about 40 conflicts throughout the world, from the Mideast to Africa to South Asia to East Asia--or the pain that will be necessary to institute eventual economic recovery, similar to what we experienced in the early 1980s. All it will take for Democrats to take the Presidency in 2020 is a GOP administration unable to handle the SHtingTF--and that shouldn't be too hard to ensure, they think, all they have to do is first get a DemLite a la Jeb in the WH if they can, and if they can't, obstruct and obfuscate for four years, and not make the mistake they did 36 years earlier by letting Cruz be Cruz or Walker be Walker, the way their forebears let Reagan be Reagan.
Only a truly narcissistic Democrat would want to be "Obama's third term," and the only two truly narcissistic Democrats are Obama, who isn't running, and Hillary!, who is. (Even Warren, who desperately wants to be President while saying she doesn't, a la Toranaga-sama in Shōgun, isn't stupid enough to run in 2016, unless she can create the illusion of being drafted to take the point for the sake of the unit.)
JMHO