Matt Yglesias asks somewhere what would have been the result if Comey's letter had come out earlier and Trump's video had come out later? The moral is the effect of contingency.
Along somewhat different lines, what would have happened had Obama opted to put immigration reform first, and health care second back in the first days of his presidency? I could argue that there was a deal to be made on immigration (almost had one in the last year of GWBush's presidency) that would have reduced the heat the issue had this year. If he'd then failed to pass Obamacare, the Tea Party uproar in 2010 might have been less effective, meaning less energy for the populist resentment this year. And having passed immigration reform might have improved the Latino support for Clinton this year.
Of course, with all those what-ifs, Trump might not have become the nominee.