The answer is likely someone most of us haven't even heard [much] of. It will probably someone who is holding office at that time (in 2020). Chris Christie' term ends in 2018, and John Kasich's and Andrew Cuomo's ends in 2019. Unless Kasich is the Republican nominee in 2016, his career is likely over after 2019. O'Malley and Chafee have been out of office for a year, which I believe hurts their chances at receiving their Party's nominations this time around, making it even less likely that they will come back in 2020. It is only the establishment figures with massive war chests and name recognition, like Jeb Bush and Mitt Romney (both out of office since 2007), and Hillary Clinton (out of office since 2013) that really have a shot. Unless Cuomo or Kasich make headlines due to some unforeseen event in the next five years, it is unlikely either of them will be on the ticket. And the same goes for Senators like Rubio and Paul. If they aren't on their party's 2016 ticket, and don't win re-election in the Senate next year, they're both done. The pressure is on Cruz (assuming he's not on the GOP ticket next year) to win re-election in 2018. There are too many unforeseen events at the moment for me to make a comfortable prediction.
Andrew Cuomo can be elected again in 2018, Paul can run for Senate/House/Governor(2016/2018/2019), Kasich can run for Senate, Rubio can run for Senate/Governor(2018/2018), and O'Malley can run for Senate or Governor in 2018.
