Blizzard will introduce Hero Pools in Overwatch's Competitive Season 21, meaning that different heroes will become unavailable every week They also provided details on how it will work and how the idea came to be.
Overwatch tends to get into its dull state when a meta settles in and Blizzard are looking to shake things up by introducing Hero Pools. This means that each week a total of four heroes will be unavailable for everyone. One tank, one support and two damage dealers will be removed from the pool, in hopes of people picking different compositions.
According to Scott Mercer, the idea behind the pools was to shake up the meta as changing it was hard even after the switch to 2-2-2 role queue.
Considering that there are more than one barrier tank and a total of 16 damage heroes, it is hard to imagine this amount of unavailable heroes having much of an impact. Then again, if too many heroes were banned, it would probably cause too many players to not be able to play their favourites too often.
That said, it is not impossible for a hero to be removed two weeks in a row. Furthermore, the Hero Pool will apply only to Competitive so those going for Quickplay or any of the Arcade modes will be completely unaffected.
As for Overwatch League, pros will have some control over the heroes that get removed in each interval. Mercer stated that heroes that get picked more than 10 per cent of the time will be eligible for removal in the next round. Technically, a team could opt to pick a different hero in order to avoid one getting more than 10 per cent pick rate but it remains to be seen if this is feasible in practice.
Mercer also chimed in on why the choice of bans isn't simply given to the players and the reason is apparently that Blizzard don't want the players to stare at UI for five minutes before starting a match. It's a bit vague whether that argument could hold up if each team was given 15 seconds per ban. That would result in the banning phase being over in two minutes at most.
Anyway, you can check more of Blizzard's insights on the upcoming Hero Pools on the official website.