Hillary's gotten incredibly lucky with having deeply flawed Republican opposition this year. If they can come-up with someone good in 2020, she's going to have a challenge on her hands, especially if the general public thinks it's finally "time for a change" after three Democratic terms.
I think the only Republican candidate who would prompt a clear "time for change" stance from the Republican base is Cruz. HE is the embodiment of the "not conservative enough last time" double down, and him being the Republican candidate only to get trounced in the general might prompt some "we went too far" introspection.
Trump is problematic in terms of him being the nominee leaves too many excuses for why he might lose the general. For example, not "Christian" enough, no political experience, he is a billionaire the common man can't relate to, etc. There would be enough questions that I could see them going back to the old strategy rather than forward to a new (and more moderate) one.
Rubio becoming the nominee then losing the general still leaves open the "should have gone more conservative with Cruz" line of thinking, especially given Clinton vs Rubio would probably end up relatively close (closer than Clinton vs either of Trump or Cruz anyway).
Regardless of who ends up being the nominee, my expectation is to see some sort of group splinter off the Republicans in some doomed in the short term way. Either the disgruntled Tea Party crowd will break away to do their own thing, or the frustrated "libertarian" crowd will. As a result, I'm not sure the Republican party can recover in time to make a strong showing in 2020.