Blizzard has considered the possibility of a pick and ban system for Overwatch, should the character roster become too unwieldy.
Talking at a Q&A attended by GameSpot, lead hero designer Geoff Goodman said, “We talk a decent amount about
–that, and what’s paired a lot with it is hero bans, hero picks, things like that because they’re both kind of related to roster size.”
Pick and ban systems are most commonly used by MOBAs such as League of Legends and Dota 2, but are applicable for any game sporting a selection of heroes with special abilities. Such a system allows opposing teams to ban certain characters from being picked at the start of a match, which is handy for denying a hard counter to a character your team plans to play. The system is typically only used for ranked/competitive matches. In the world of shooters, this system has most recently been adopted by Rainbow Six Siege.
Source: IGN.com Overwatch May Feature a Pick and Ban System in the Future