If Trump wins in November, and decides that the actual workload of governing as President is too much for him, he may rest on his laurels and retire (he will be 74 in 2020, after all) and work to get his VP (likely Christie, Sessions, Corker, Fallin, or Huntsman) elected.
Wikipedia mentions Justin Amash, Chris Christie, Tom Cotton, Ted Cruz, and Marco Rubio as potential Republican candidates, as well as Cory Booker and Andrew Cuomo on the Democratic side. There is also, of course, Kanye West, although I doubt he'll actually go through with it.
2016 has been a populist primary season for the Democrats. Cory Booker and Andrew Cuomo represent a wing of the Party (the corporate, "moderate," DLC Third Way Democrats) that are dying. They may run, but their chances of winning, at least in a general election. Hillary Clinton, if she wins in November will be the last of the Third Way Democrats to hold the Oval Office. I think populists like Elizabeth Warren, Sherrod Brown, Al Franken, and Alan Grayson are more likely to run and succeed. I am clueless about the Republican side, but whichever Party Kanye West decides to run in, I actually think he has a good chance, assuming the momentum that exists with Sanders (an insider who plays the role of outsider), Trump, and Carson (both complete outsiders) holds up. West is the next reality show star to get famous by using free media publicity and outrageous statements. Underestimate West now, but if he follows the Trump model, he may be a real contender.
