That third party thing
But if Cruz or especially Trump gets the Republican nomination, there might be room for a Center-Right third party candidate.
The only question would be who. Lindsey Graham? Mitt Romney? John Kasich?
How about... Jeb Bush?
Hard to find an obvious fit. But the niche is there if anybody wants to grab it. Unless, of course, Rubio gets the nomination. If that doesn't happen, though- if it's Trump or Cruz- I, for one, would be interested in supporting such a candidate.
Incidentally, I don't buy the notion of Trump running as a third-party candidate if he doesn't get the Republican nomination. He'd be wasting his money, and that doesn't sound like Trump.