Let us say that Trump wins, but only narrowly(270 to 268, courtesy of Maine's district). However we then see something we've never seen before- faithless electors tip the election to the other side(there are still 21 states in which it would be entirely legal for them to do so). If anyone was going to provoke faithlessness it would be Trump, after all, and there has already been one elector making noise about how she might be faithless(
https://www.washingtonpost.com/news/the-fix/wp/2016/08/03/how-a-faithless-elector-in-georgia-could-cost-donald-trump-an-electoral-college-vote-explained/). Only 2 need to defect.
Importantly this means that Trump hasn't been discredited by defeat- he won, and it was
stolen, will be the narrative even among his Republican opponents(and indeed most independents, and even many Democrats, will see it that way as well).
Now, consider what the following 4 years will be like. We're at the point in the business cycle where we should expect a downturn sometime in the next four years... not only that, but there are suggestions of a potentially imminent financial crisis(
http://moneyweek.com/us-house-prices-cause-of-the-next-financial-crisis/) as well. Any positive action Hillary might take to alleviate things will be stoppered by a Republican congress that is even more implacably opposed to her then it was to Obama(if that's even possible)- she "stole the election" after all.
So, the 2018 congressional elections are a landslide favouring the Republicans(as always it is the party holding the presidency that is blamed for the problems of the day). Trump kicks off his third presidential campaign the day after said elections, proclaiming them a rebuke of the illegitimate Clinton presidency, and proceeds to win the Republican primaries much as he did in 2016(if anything his support should be higher given the circumstances of his defeat and the public hate for the Clinton presidency).
Clinton chooses to contest the 2020 election, and wards of any primary challenges. Trump is then able to achieve a clear victory in the presidential election, mostly through increasing his margins among white voters and increased defection to third parties by Clinton voters. This is coupled with further gains for the Republicans in the senate.
Trump is thus inaugurated in 2021, and the Trumpification of the Republican party is far more secure then it would have been if he had won in 2016- after all, he has won not 1 but 2 primary campaigns, there have been 4 years of bellyaching about how he was the legitimate winner in 2016 and he is likely to benefit from a cyclical upswing over the course of his first term. Courtesy of this and a poor Democratic challenger in 2024 he wins reelection...
Nightmare scenario? Well no, the nightmare would be him kicking of WW3 in his first term thus rendering the 2024 presidential election a non-issue.
