Based on what I've read and seen, a President (Heaven forfend) Trump would do whatever he wants to do at the time. I do not believe that he would feel bound by any promise he has made at any point during the campaign (no, not even to build a wall between the US and Mexico--though that shouldn't lead one to feel relieved).
I think we would be well-advised to consider certain aspects of Trump's style or modus operandi, as well as elements in his "The Art of the Deal" (see http://www.wsj.com/articles/trumps-campaign-blueprint-his-own-book-1456949133). For example, Monica Langley writes,'One key strategy [of Trump's] is to "fight back," and to do so "very hard" against "losers."' (WSJ 3/3/16, p. A1).
Here is my list of 'anticipations' or, better, worries, about what a President (heaven forfend) Trump might be like based on what he has done or said to date:
He appears to value action for action's sake, where action means directing others to do one's bidding.
He defines leadership as imposing one's will on others (Paul Ryan must cooperate with him or 'he's gonna have to pay a big price, OK?'). He would be likely to 'lead' in this fashion.
He seeks enemies ('losers'). This appears to be central to his functioning.
He appears to value strength for strength's sake.
He likes to leverage resources. It appears to me that he sees the election as a way to gain control over the entire USA to use it as he would.
He likes to intimidate. He appears comfortable in his campaign with threatening opponents with violence and the press with prosecution.
He can be remarkably gracious when it suits him. His intimidation and graciousness are two sides of one coin: Complementary ways to bully and dominate others.
He is not likely to tolerate dissent. (See for example: Mr. Trump directed his venom toward the New York Times, which he called “dishonest” and “the absolute worst,” and to The Post and its owner, Amazon founder and chief executive Jeffrey P. Bezos. “I have to tell you, I have respect for Jeff Bezos, but he bought The Washington Post to have political influence,” Mr. Trump said. “He wants political influence so that Amazon will benefit from it. That’s not right. And believe me, if I become president, oh, do they have problems. They’re going to have such problems.” https://www.washingtonpost.com/opinions/is-mr-trump-a-threat-to-democracy/2016/02/26/d02ed222-dcd9-11e5-925f-1d10062cc82d_story.html).
He is expert at discomposing social structures (see how he has turned Republican debates into a circus). He could do that to the Congress and Supreme Court
He appears to tolerate experiencing failure and leaving behind a string of failures.
He is unlikely to take responsibility for failures or for failing to live up to his expectations. (He blames other countries for the facts that his campaign sells merchandise that is made in other countries, as reflected in the transcript from the recent debate, http://www.nytimes.com/2016/03/04/us/politics/transcript-of-the-republican-presidential-debate-in-detroit.html?_r=0).
He likes to put his name on structures.
Institutions that he might not allow to stand in his way include:
The Constitution
The Legislative Branch
The Judicial Branch
The Army
The Laws of the Land
Alliances and commitments to foreign powers
For an example of his stated disregard of the laws of the land, consider this from the Th 3/3 debate (http://www.nytimes.com/2016/03/04/us/politics/transcript-of-the-republican-presidential-debate-in-detroit.html):
"So what would you do, as commander-in-chief, if the U.S. military refused to carry out those orders?
"TRUMP: They won’t refuse. They’re not going to refuse me. Believe me.
"BAIER: But they’re illegal.
"TRUMP: Let me just tell you, you look at the Middle East. They’re chopping off heads. They’re chopping off the heads of Christians and anybody else that happens to be in the way. They’re drowning people in steel cages. And he — now we’re talking about waterboarding."
In sum, I would anticipate that a President (Heaven forfend) Trump would see the USA as his resource (his by right, in his view) to use as he sees fit.