Interesting question. All of the above, of course. But I wouldn't blame the rest of the GOP field, though, just most of them.
The most immediate threat is illegal immigration. It's existential. Trump has put it first. Will he continue to grasp and act on the fact that we must be decisive and cheerfully do what is (to liberals only) unthinkable. If he's truly for "touchback amnesty," he doesn't get it.
The second-most important threat is Moslem invasion. He seems to get that, and in the right priority.
The over-arching threat, which is both existential and eternal, is the destruction of our culture caused by immorality and the lack of religious sense and self-discipline. This has led to the destruction of both family life and the economy. In a nutshell, young men with a sense of purpose and families to feed don't hang out playing video games in their parents' basements. These young men inspire young girls to look forward to marriage. These young men sire legitimate children who know how to work, love, and pray. Such children, our real future, are being born in certain enclaves, but it should be happening all over America, and it's not.
The last threat is the ultimate cause of the first two. Does Trump get that? Does he have the attention-span to even grasp it? I hope he does. May his Presidential journey make him realize it, and be the leader he should be.