The prospect of another Clinton-Bush race should invite a populist rebellion, challenges from both the right and left. A democratic socialist, Bernie Sanders, may pose a serious challenge to Clinton from the left, and a Donald Trump or Rand Paul challenge to Bush on the right. If they lose in the primaries, would they mount independent, third-party challenges? Seems likely.
With economic frustration and, in some parts of the country, stagnation, the country is ripe for a populist rebellion.
If neither Clinton nor Bush face serious challenges within their parties, 2016 could be quite a boring and stagnant cycle, on a par with 1988 or 2000, predictable candidates saying very predictable things, and many voters detaching or disengaging from what is supposed to be participatory democracy. That would not be healthy.