The BBWAA will still have some players backlogged that year; it's what they do. Yeah, I'm pulling us further from the OP by asking - how long with players like Curt Schilling remain on the outside? It becomes the what-about game. Well, if Mussina gets in, then "what about" Schilling? Without straying into politics, we know that there are things besides the numbers that go into the voting games. Or is there another reason why Eddie Murray, of the 500/3,000 club, "only" received 85.3% of the vote besides his less than favorable reputation amongst the writers? So short story, long, I don't think there will be an empty 2021 class. It just won't (shouldn't) have any first ballots.
There are also the Era (formerly called Veterans') committees that may add to the list. 2021 is Golden Days (1950-69) and Early Baseball (Before '50). 2010 had 1 from the BBWAA, 2 in 2011, 1 in 2012, and 0 in 2013 so an empty class wouldn't be unprecedented.