Batman, as a character, isn't someone who often expects to find happiness or victory. In his never-ending war on crime, he's risked death countless times. It's not unreasonable for he and his allies to imagine that someday, his mission will be the death of him. But what if that demise doesn't come for decades?
What happened in the possible future of Tom King's Batman run where Bruce Wayne actually grew old and passed away peacefully -- and where are things in the timeline in the upcoming Batman & Catwoman?
In this timeline (which was teased frequently through Tom King's run on Batman) Bruce and Selina have been revealed to have lived a quiet and peaceful life. The Bat-Family grew around them, with other members slowly adding to their ranks. Some found happiness with each other, such as Duke and Claire (aka Signal and Gotham Girl respectively). Their daughter, Helena, grew strong and clever like her parents -- with Selina even fighting crime while pregnant with her. Helena accepts the responsibility of her family name, becoming the Batwoman of this era and continuing on Batman's battle against the criminals and monsters that threaten the people of Gotham City. But after decades of a happy marriage, one trip to the doctor shattered Bruce and Selina's perfect life.
Bruce is revealed to have contracted a particularly lethal form of cancer -- later revealed to be the end result of a long-ago battle with the radioactive villain Doctor Phosphorus finally coming to fruition. Despite the best efforts of all his oldest and most deadly villains, it was the usually unassuming C-lister who ended up being the final death of this version of the Dark Knight. Despite approaching Zatanna -- who in this future is still a major and active figure in the magical community -- there's nothing Batwoman can do to help rescue her father. Another hero confirmed to still be alive is Flash -- who Bruce jokingly tells his wife to find after his death, so the Speedster can find another Bruce Wayne from across the multiverse to take care of her.
As his body gives out, Bruce ends up conscribed to a bed, unable to complete the impressive physical feats he once could. Although the entire extended Bat-Family makes sure to visit him before his passings (including older versions of Nightwing, Batgirl, Red Hood, Robin, Signal, Gotham Girl, and Carrie Kelly), it's only Catwoman who's with when he finally slips away one morning, holding her hand as he dies peacefully in bed. It's a somber moment for Batman and Catwoman, the final quiet ending for the hero and the loved one he leaves behind. But it's also proof that Bruce Wayne doesn't have to forsake the world to find happiness. While he may never fully give up his mission to help save Gotham, there's a way for him to find happiness with others, to love and be loved in return despite his mission.
There's a version of Batman who doesn't die in the heat of battle but rather survives to see his family grow and expand and thrive before he passes away like so many men before him and countless after. The upcoming Batman/Catwoman maxi-series will delve further into this time-period, possibly exploring how the rest of the future has survived without the original Batman fighting for it. Selina will try to accomplish a long-ignored mission for revenge against a mysterious subject and may find some vengeance before she joins her husband in whatever comes next.