Blizzard has revealed a number of new features coming to Overwatch that are set to arrive as part of Patch 1.45, which is scheduled to land at some point in early February. These changes are mostly focused on allowing more frequent and impactful rebalancing, to "deliberately change the meta" and experiment a little.
A large part of that process will be the new 'experimental card', a mode available on PC and consoles that will allow Blizzard to test changes to characters, maps, and even modes.
"Unlike the PTR, the experimental card is not for bug testing," Blizzard explained. "Instead, it will be used to experiment with balance updates, game modes, or changes to rules that we are investigating internally. While some of these features will reach live, the content within this game mode is not guaranteed to be released."
Another new addition for Season 21 is the Hero Pool. This feature will disable some heroes on a weekly basis, allowing the developers to better focus gameplay around specific styles. Blizzard will be testing the Hero Pool during the coming season, and if they find it to be a negative addition, it might not return for Season 22.
You can find more details on everything above by hitting this link.