Rand Paul, Marco Rubio, and Scott Walker are the three candidates in this cycle who are most likely to run in 2020 should Jeb Bush lose the general election.
If Bush wins the general election, by 2024, Paul will likely be the only one still in office - Walker would have great difficulty winning a third term and Rubio is not running for re-election to the U.S. Senate, though there is speculation he will run for Governor of Florida in 2018.
I don't think Walker will wait 4 years. What is he going to do doing that time? I'm pretty sure Rubio will run for governor. Walker might run for Senate.
Walker will still be Governor of Wisconsin until 2019, so if he runs in 2020, he would begin running in 2019.
Rubio would only be out of office for 3 years by 2020, Jeb Bush has been out of office for 8 years now. Reagan was out of office 5 years by 1980.
But I am confident Jeb Bush will win and be the GOP nominee in 2020.
