For some reference – three years before winning their first Presidential elections:
Bill Clinton, in 1989, was in his third term as governor of Arkansas. There’d been some speculation about him running for president in '88 (when Cuomo declined to run, and Gary Hart dropped out), but he’d never entered the race. His biggest national exposure at that point had been being a keynote speaker at the 1988 Democratic Convention, and going way over his allotted time.
George W. Bush, in 1997, was partway through his first term as governor of Texas, part owner of the Texas Rangers baseball team, and the son of (and shared a name with) a former president. Nationally, he was probably better known for the second and third items from the above list at that time than he was for the first.
Barack Obama, in 2005, was a year into his term as a U.S. Senator (an election in which his victory was, in part, due to the Republican nominee having to withdraw due to a sex scandal involving his actress wife). He had given a keynote address at the 2004 Democratic Convention, and was seen as a rising star in the party, but was also very new to the national stage.
In short: at that juncture, 3 years before the elections, few people would have seen Clinton, GWB, or Obama as their parties’ favorite or lead candidate, though all three had a certain level of national awareness (particularly GWB, due to his name, and governorship of a large state). Based on their roles in the previous conventions, Clinton and Obama were clearly being groomed for bigger roles, but when the races actually started, neither of them were seen as the top challengers.
So, my guess is that the Democratic nominee for 2020 will be someone that you’ve probably heard of today, but may not know much at all about.