Never going to happen. Assuming the establishment cannot swing it to Rubio, the designated Heir Apparent, then we'll see a continual string of country club Republicans declaring that conservatives have to find "another choice". Because they are much closer to the Hildabeast than to Trump. Which is about the only reason anyone would ever vote for Trump, far as I can see. Well, that and he isn't Hillary. However, not being Hillary is not necessarily the same as being better than Hillary.
Believe me, nothing makes me happier than seeing someone like Trump running in the Rep primary. Reps abandoned fiscal conservatism, along with just allowing crazy religious dogma into the party, and are reaping the benefits of having to start an election cycle pandering to their base. I feel bad in a way, because 'We'll give you free shit!' is so effective, it's hard to be the adult in the room vs the Dems, but that's what the Reps are
in theory supposed to be. When you fail year after year, decade after decade, at what your role is supposed to be, it eventually starts to erode your base (and the base is eroding anyways, because of 'It's free!' loss/indoctrination).
The Rep party really needed to have a burn it to the ground moment when Bush II left, they've just prolonged the inevitable and it's leading to Trump being their candidate. So be it. (that Trump has some good ideas/pays lip service to them, all the better)
If by some miracle Dems actually grow a 3rd brain cell and rub all three together at once (basically an impossibility, but, they're supposed to be creative, so there's a chance) and nominate Bernie, that would be amazing. I know lifelong Reps who have zero problem (so far) voting for Trump but if Bernie is nominated are a lock for him. The electorate is looking for someone who is genuine and not fake, which is why Trump is actually doing so well. Billary? Her own party doesn't like her (except for the party jizz slurpers), she's about as far from not fake and genuine as one could get. A very interesting election!