I think it is a given that Romney got elected to the senate with the sole purpose of positioning to run for POTUS. I was thinking he would go in 2024, but he is arrogant enough to think he can beat Trump. If he runs against Trump in 2020, he will lose. He is an established loser, and Trump will make him road kill.
There is only one circumstance that would make sense for Romney running in 2020. If there is a plan to JFK Trump in 2020, if Romney is running, he would most likely get the nomination. This has been my fear for Trump since he won in 2016. He has made many enemies, particularly the intelligence folks and military, who could take him out any time they choose. His pulling out of Syria might just be equivalent to JFK's executive order to withdraw from Vietnam. That action triggered his assassination by the military industrial complex.
I think Trump is being setup for another JFK moment.