I wrote a timeline a little while ago about this, although I abandoned it soon after I began it. I positioned Trump to still run, although he would do so as a Democrat, a social democrat who is oddly against immigration. Obama running again could probably happen, although it would depend massively on Romney's success, as much of Obama's appeal would be due to a kind of buyer's remorse, which was partially the appeal of Cleveland in 1892, and the near-candidacy of Jerry Ford in 1980. Hillary, though, would almost certainly have run against Obama, and I don't think Biden could be totally counted out. If you add a socially democratic, populist Trump, probably Bernie Sanders, and any other more left-wing figures, and it will be a split field led by a defeated President who would have been seen as largely failing in his bipartisan attempts, an extremely controversial former First Lady-turned-politician, an older gaffe-prone former Vice President, a brash, big-mouth billionaire without any experience, and a somewhat elderly man offering radical changes. All of these people offer different things and could possibly end up tearing up the party, especially if the Republican Party unites around Romney, with added points if he is even moderately successful. Honestly, with the economy being the way it was at that point, it would be more difficult for Romney to fail than to succeed.
(Goddamn it, I got beat to it, but the point still stands)