Black Ops 4 is noteworthy as the first Call of Duty game without a single-player campaign. This move was met with negative criticism at launch, but the game proved it could survive the lack of single-player content. You could argue that the multiplayer-centric approach of Black Ops 4 paved the way for the current and ongoing success of multiplayer-only Call of Duty titles like Warzone as well as its upcoming sequel, Warzone 2, and the mobile spin-off, Call of Duty: Mobile.
However, the Black Ops games generally had well-received campaigns. Because of this, it still stings that Treyarch didn't continue with a future-centric campaign after going back to the 80s with Black Ops - Cold War.
If we believe the very elaborate leak on Reddit, it appears that Treyarch considered having a robust single-player campaign for Black Ops 4 that would have done some interesting things.
In summary, u/PurpleToaster20 claims that Treyarch originally planned Black Ops 4's would have had a 2v2 campaign mode called "Career". Meanwhile, in terms of narrative, Black Ops 4 would be set years after Black Ops 3 in 2070. In this reality, players would work with either the Free People's Army or the World United Nations. Although short, each mission would have gone into detail about what happened to the world and each faction's motivation. In addition to this, the campaigns would have offered a variety of gameplay experiences like pitting each side against each other and having unique missions depending on which side you chose.
All things considered, the shuttered single-player experience for Black Ops 4 all sounds like it could have made for quite the game. Unfortunately, it also feels ambitious and risky.
It's a shame Black Ops 4 didn't get a single-player campaign. In a different world, Treyarch might have released Black Ops 5 instead of Black Ops - Cold War in 2020. If it's any consolation, the leaker claims they will upload a video detailing all the info for 30 minutes on August 18. So, you'll want to circle your calendars for then if you'd like more juicy intel about what could've been for Black Ops 4.
Speaking of remembering dates, you'll want to remember September 15 as well for Call of Duty Next. The upcoming showcase will reveal everything there is to know about Modern Warfare 2, Warzone 2, and Mobile, as well as possibly other upcoming Call of Duty games. The event is scheduled just a day before the beta period for Modern Warfare 2.