After getting into Overwatch and reading the backstory and character stories, I've come to see what Blizzard did with Overwatch as something really clever. The Overwatch we have as it is, with 21 (now 22) playable characters, and decades of backstory, is essentially something like King of Fighters XIII or Street Fighter IV, except it happens to be the first game.
Looking at the history they're wrote up for the game, we can easily see the framework for concepts of "Overwatch 1" for example, which could have been a shooter made in the early 00s, with a simple story campaign about a special ops team, and multiplayer with 5 character classes. Then there's "Overwatch 2" which would be after the first Omnic Crisis, and instead of just fighting robot enemies you have wider global issues to deal with, and with the "expansion" of Overwatch there's the excuse of adding a bunch of new characters and factions.
Finally, with Reaper's betrayal and the destruction of Overwatch, we get to the third major period where new heroes and villains rise from the ashes and answer the call in an uncertain future. That's where we are now. Pretty cool imo!