Batfam and the Lazarus Pit
This isn't my idea, and I honestly can't remember if @frownyalfred or @bruciemilf came up with this idea first, but it's been living in my head rent free and I need to get it out.
There are Lazarus Pits under Gotham, even contributing to why the city's so cursed. The Bats have each used a Lazarus Pit at least once (maybe not Spoiler and Signal, but I'm not sure). I'm fairly sure it's also canonical that the more you use it, the more prolonged the effects are and it kinda changes you incrementally but permanently.
Enter the Batfam, who train like crazy and are Olympian-level athletes all on their own, and using the Lazarus to help each other on death's door. (They don't tell each other when they do this, and they think it's never been done to them - except Jason, he can always tell - but they also don't share when they've done it to others.)
As time goes on, everyone gets older, stronger, more proficient at their jobs. Some take on younger teams, some proteges, some fly solo or stick around home. It's one of the OG Leaguers who points it out one day when they're having a civilian lunch - probably Ollie or Hal. In my head Bruce is one of, if not the youngest, OG Leaguers. So it's not crazy when Clark or Barry start to wrinkle around the eyes or get grey in their hair well before Bruce would. Bruce is also a public figure - he's got appearances to keep up.
But then ... Bruce is over 40 and his hair is still as black as it's ever been. His wrinkles are from his scowling and focus, only crinkling around the edges of his eyes and mouth a little bit. Idly, Hal wonders out loud if Bat's eyes have always been so dark, almost like coals. Barry notes that Bruce is way bigger than he used to be, that he bulges the suits he wears to galas with his size. Ollie, who's own beard is greying, bitterly points out that even Bruce's stubble is still black.
And suddenly, Bruce hits 50 and he's still thick-chested and dark-haired. The other Bats only seem to get more and more ... more. No non-meta can spar with them anymore unless they hold back and they seem to have endless stamina and pain tolerance. Clark and Diana think nothing of it, but the other fully human Leaguers start to wonder what's really going on. If maybe Bruce had lied to them. But J'onn swears that in their own minds, all of the Bats fully believe they're human. Aside from the suspicion, there's no reason to believe otherwise. They still bleed and scar, no matter how beautiful they seem to remain. No matter that Bruce keeps going and going and going even after others his age have been retired for years.
Fed up one day, Ollie asks him how he does it. Dermatologists? Botox? Just For Men? Bruce snorts and rolls his eyes. He smirks, "Good genes, I guess."
Bruce is nearly 70 when the grey creeps into his hair and his body starts to slow. His children are still active as ever, and when Tim takes up the Batman mantle, Bruce retires to train new heroes. One young hero complains that Bruce hits like one of the Amazons she trained with but he only responds that he's never stopped being the Bat even with tbe cowl off.
And then ... Leaguers start dying. Gradually, age takes them one by one and they's succeeded by the heroes they've mentored and the children who followed in their footsteps. Grey is starting to creep into Dick and Jason's hair, but they're still as vibrant and lively as ever.
It's only when it's just him left of humans of the original group that he actually considers why. If anyone should have died young, it should have been him. And yet, he's nearing 90 and not nearly as withered as some of them had been when they passed away. The vitality he's always attributed to his genetics and continued activity can't explain why all his children remain as beautiful and capable all their lives as he has. So one day, he gathers his wayward children down in the cave like they used to 50 years ago and asks them to be completely honest with him.
"Have any of you used the Lazarus Pit on anyone here without telling them or reporting it?"
The silence he's met with is deafening.
