Originally I thought his campaign was performance art. I actually still think a lot of it is... but now I feel pretty convinced that the method he's honing will take him at least to the Republican nomination. Once that happens it's not much further of a stretch to think he can present a strong challenge to the Clinton machine.

Trump's campaign is pure, shameless marketing and hard-selling to whoever he needs to at any point in time. He has a few core beliefs that he's had for a while, but he's not an ideologue. His campaign's core message is about driving out corruption and incompetence in our federal government, the rest is largely signaling. Scott Adams has a great blog series about Trump's persuasion techniques

He's always been anti-interventionist, but he knows who his audience is. 4 months ago he would just say "We will build the military so strong your head will spin!" and in the past 2 months started adding "And we don't need to be in every conflict, it can be so cheap!".

He's controversial in his messaging and it plays well to conservatives, but I think once he gets into general election mode you're going to see a different animal.

Some positions that he's already supported that are going to become more of a focus:

Non-interventionist foreign policy (possibly even less interventionist than Sanders)

Legalization of medical marijuana at federal level

Little religious baggage, neutral on gay rights (originally supported it before Hillary), and social conservatism in general

That's just off the top of my head.