The 2016 Democratic Primary will be between Brian Schweitzer, Martin O'Malley, Andrew Cuomo, and Mark Warner. Hillary running depends on who wins this year, so if Romney wins, Clinton will probably run because she knows she could win. It's harder to follow a two termer. I think the Democrats will probably go with Clinton or O'Malley. If Clinton doesn't run though, Cuomo would be better than O'Malley in the general. Unless we nominate someone that'd turn off indies, like Elizabeth Warren, we could take Romney down easily.
Hillary didn't want to run in 08 though, and even hints that it was Bill who goaded her into running:
http://www.dailymail.co.uk/news/article-2071598/Hillary-Clinton-doesnt-want-President-just-wants-sleep-says-husband-Bill.html