I think that the worst he will do is debase the political process and the divide in the country to even worse levels than they are now, perhaps to even an unfixable level. He has already demonstrated that being an asshole who caters to a minority and demonizing the other side is an effective strategy and that just plain not caring about the truth isn't a negative, so this strategy is going to be emulated again and again.
I disagree that division will be his greatest impact. He only got elected because their are already very deep divisions that stem from the very core incompatible values and worldviews that people have which are no more reconcilable than science and religion.
Conservatives already understood this and have for decades cast liberals as the enemy, which is in fact accurate. Liberals are the enemy of the core values of conservatives, and vice-versa. Liberals have been trying to kid themselves that "we're all one nation and the same at heart and want the same things." No, no we don't. Liberals tend to assume that its just a few bad leaders misguiding some voters who can be reached with the right message. This naive view is partly due to the liberals rose-colored view of human nature and over-emphasis on cultural and socializing influences while ignoring factors that are either innate or so ingrained at an early age that they are not changeable.
The one potential positive by-product of his election might be that more leftists and radical liberals who have been fool enough to view Hillary and the Dems as too much of an enemy to form an alliance with, will realize who the real enemy is and just how much much more "evil" they are than the "lesser of two evils" they chose not ally with and thus ensured the greater evil would win.
I actually think enough people have already learned this lesson that if the same election were somehow held today, Trump would lose the electoral college and not just the popular vote.
BTW, I used quotes around "evil" because I don't believe in evil in an absolute sense, just in that they are prone toward ideas that are anti-thetical to moral and scientific progress that is in the interests of most of humanity. As for "demonizing the other side", you cannot accurately asses Trump's words and his goals without it "demonizing" him. To the extent that demons exist in a metaphorical sense, he's it. Nothing Hillary ever said about him, demonized him as much as his own words did. That speaks to the issue that "demonizing the other side" isn't the real problem, so much as failing to recognize who are the real demons and who are less than ideal players that we shouldn't demonize but rather ally with to defeat those demons.
The divisions are real, have always been there, and cannot be repaired only masked. We need to recognize where the real core divisions are, and form alliances across more minor divisions to ensure that the segment of society that will always be ready to give into the worst aspects of their nature have as little political influence as possible.
