There's a cliché saying about presidential primaries: Republicans fall in line; Democrats fall in love. In Republican circles, their nominating process has historically been very predictable, usually recycling the candidate who was runner-up last time. What Mitt Romney, John McCain, Bob Dole, George H.W Bush, and Ronald Reagan all have in common is they all unsuccessfully ran for president at least once before securing the Republican nomination. Democrats, on the other hand, tend to do some soul-searching on who they want to nominate to be their standard-bearer, which makes their primaries historically a lot more unpredictable and allows for insurgent candidates like Barack Obama, Bill Clinton, Michael Dukakis, and Jimmy Carter to come up out of nowhere and capture the nomination. Going into 2016, at first glance it appears that the roles have reversed: Republicans are wandering aimlessly through the political wilderness in search of a savior after back-to-back losses while Democrats are all but set on Hillary Clinton.
If history repeats itself, former Pennsylvania Senator Rick Santorum is next in line for the Republican nomination, as he was runner-up to Romney in the 2012 Republican primaries. He is strongly considering running in 2016, but I highly doubt he'll be the nominee unless the Republican Party is interested in being obliterated in a Goldwater-esque shellacking. This leaves their field wide open for 2016, with some from the 2010 generation (i.e. Rand Paul, Marco Rubio) already making moves. As of yet, there's no clear front-runner.
On the Democratic side, everyone's waiting on Hillary Clinton, and until she makes a decisive statement on whether she is running or not, the field is frozen and donors are stuck in limbo. Clinton has consistently been by far the front-runner and the overwhelming favorite. If she does run, the Democratic primaries in 2016 would likely mirror the primaries in 2000, where Al Gore ran virtually unopposed save for a weak challenger in New Jersey Senator Bill Bradley, and went on to win every primary and caucus. The reason Clinton is unlikely to face any serious challengers in the primaries is that many of the possible Democratic contenders for 2016 share the same donor base as her and are loyal to her. There would be little incentive for anyone else to run. New York Governor Andrew Cuomo shares the same donor base as Hillary Clinton — a donor base that is far more loyal to her than they are to him. Maryland Governor Martin O'Malley is loyal to the Clintons, and he would have little incentive to run but to increase his name recognition (the silver lining behind every Republican who runs for their party's nomination and loses the first time around), but no one wants to be remembered in 2024 as the guy who ran against Clinton in 2016. Vice President Joe Biden consistently polls in a distant second place to Clinton. If she didn't run he'd be the front-runner according to the polls, but it'd be very surprising if he chose to run against Hillary. It would also be surprising if his good poll numbers in the primaries aren't due to the universal name-recognition he enjoys rather than substantive support.
Of course, three years is an eternity in politics, and it is impossible to accurately gauge the political landscape of 2016. Keep in mind that at this stage in the game leading up to 2008, Hillary Clinton and Rudy Giuliani were considered the favorites for their respective party's nominations, and 2008 was looking like it might be the 2000 New York Senate race that never happened. Should for whatever reason Hillary not run, I'd keep an eye on Maryland Governor Martin O'Malley as a dark-horse candidate in the race. He's not on many people's radar just yet, but he is a two term governor with clear national ambitions and was the chair of the Democratic Governors Association. That's a very important job to have in terms of building the national fund-raising network required for a presidential run. Bill Clinton was once the DGA chair, as was Howard Dean and Bill Richardson.
It is possible for a party to simultaneously fall in love and in line with their nominee. Republicans did that with Reagan in 1980. I think it's the same case with Democrats and Clinton in 2016. They've already found their presidential soul mate while Republicans continue the speed-dating game that is their nomination process.