As far as for the Democrats. The question is will Obama be the candidate? I can't imagine anything that would prevent this from happening even if his numbers are low and continue to be low. If he is challenged, from within, then I fully would expect it to be from Hillary but I would think something next to impossible would have to happen for this to occur.
As for Republicans or potentially a third (Tea) party the field is way open. Rand Paul could very well make a run or someone we haven't heard of to this day. As for the Republican party I feel that Donald Trump is going to run and I believe will get the nod from most Republicans in their Primaries. There are plenty others such as Romney, Huckabee, Palin and Bachman that could wield power in the Republican primaries.
Of all the proposed candidates that have indicated they might run or are running I have to say I like Trump myself. I was a Reagan Democrat who changed to Independent in the late 80's. Voted Bush 41 and Clinton, Gore (Glad I lost that one) and Bush over Kerry. I made a huge mistake by voting for Obama, at least that is how I feel now, and due to the Democratic controlled House and Senate and seeing how the Democrats have done when they had power I have now changed my status to "Anything But Democrat" thanks to Pelosi, Reid, and the way President Obama has conducted himself and his office.
I feel our President is ashamed of America and is weak. I feel that he is way to inexperienced and indecisive, trying way too hard to go with the polls and whichever way the wind is blowing. I'm not too proud of the Republicans that are in office either but compared to the Democrats that are in there now they are far more preferable and less dangerous. Just my opinion.