John Kasich (remember him??)

is making forlorn noises about "challenging Trump" in 2020.  In his own way, he is just as deluded as the President, Kasich really thinks there is a Anti-Trump constituency within the GOP that can be activated during the presidential primaries.
I don't see it at all, pretty much the entire GOP has made a very bad Faustian Bargain with Donald Trump and only perhaps a truly botched international crisis or another recession will break it, and I doubt even then....
Its difficult to flank Trump on the right, because he's done such a good job of co-opting extremists, the argument has to be made on constitutional principles and our international commitments, none too popular planks come the South Carolina Primary.
I doubt Kasich will do it, for lack of courage, I strongly suspect he cannot raise the money and most of all, he is the sort of prissy politician that Trump eats for breakfast. In other words the President is good at beating up lame weaklings, and that is John Kasich in a nutshell.

No, reluctantly I am forced to the conclusion that only a serious well funded third party bid can deny Trump the votes and thwart him in the General Election. From the GOP, we can expect nothing.  Said 3rd Party Challenge will require a lot of money and credible persuasive candidate, perhaps someone can leverage the Libertarians since they already on the ballot, but that will also require a certain amount of ruthlessness...to the point of making de facto common cause with the democrats. I doubt there are any credible candidates out there, willing to assume this amount of risk in a deliberately doomed campaign.
As at the moment the only Libertarian making Presidential noises is, Bill Weld, God Help Us.