Hey so, turns out that fetal cells can transfer back to the mother during pregnancy, and often persist there for years. They're even helpful for healing and lots of other interesting changes that can occur.
Imagine an a/b/o Superbat setting, where Clark and Bruce know Clark will outlive Bruce by centuries or longer. At the beginning of their relationship, this was what made Bruce so resistant to having a relationship with Clark, he just didn't want him to spend most of his life grieving for a mate he'd (at best) only have for a few short decades.
But they're in love, and eventually Clark convinces him that he'll have Bruce or no one so they get together, and start having children. And it's so bittersweet for both of them, they're both so aware that their time together is limited, so they make the most of it, building a life together that will hopefully keep Clark warm even when all he has are memories.
And children. Lots, and lots of children. Bruce seems to have taken on the mission of ensuring that while Clark won't have him, he certainly won't be alone.
But after their youngest is born, Alfred puts together a picture book of the first Bruce-with-baby pictures with each child and he notices something... odd.
Bruce at forty looks the same as Bruce at thirty. In each, he's tired and smiling, but while Alfred can see the changes on his own face in that window of time, Bruce, if anything, looks a little better in the more recent photo.
And come to think of it, he doesn't even have any grays, and Alfred distinctly remembers buying him hair dye for the first time somewhere around thirty-seven... Hell, there are scars Alfred remembers stitching that just aren't there anymore.
Ahhh, I don't know if it's more fun for Bruce to get the book from Alfred and put it together for himself and gently explain his suspicions to Clark, or if he's caught in an explosion or something in space and left floating in the vacuum, and Clark is sure he's just going to go collect Bruce's body, but when he gets there, Bruce is just... unconscious? Hanging in a sunbeam, surrounded by debris, still strapped into his seat, metal shredded all around him, while he looks perfect--
And then Clark has to figure out how to explain? I don't know, you can probably think of something even better <3
(Please take your time with this, or just ignore it if you're not feeling an ask like this one atm <3333 )
Clark flies into the hangar in a daze. Achingly slow, he cannot bear the idea of jostling his love, even now, he touches down on his tiptoes, sinking millimetre by millimetre until his feet lay flat.
Alarms blare through the room, lights flashing red, as the hangar door begins to descend. Usually it's a quick affair. Usually they haven't just lost half of their systems to an attack on the Watchtower. Usually Clark isn't flying out into the debris, searching for his mate's body—
The door shudders into place, the red lights fade back to flickering florescents, and Arthur tears the damaged door out of it's frame to let everyone else pour in. They'd been watching through the glass, he'd seen their silhouettes beyond the soot blacking out the bay window.
They crowd around him, but not pressing close, not daring to disturb the peaceful way Bruce rests against his chest, still, unmoving.
He should've known what was happening. He'd noticed things, things that didn't make sense, but nothing could have prepared him for this, for having to go out and collect Bruce's body, thrown out in force of the explosion that took out half of the Watchtower.
His mate, gone from him in a single moment.
What could armour do against a bomb set off two feet from him?
The rest of the team staggering to intact units, sealing off the compromised areas, while Clark hung in place, staring at the black shape floating in the distance. Ignoring the screams, the shouts, the cries of pain all around him, feeling his alpha howl internally as they flew after that floating mass, knowing they were on a trip oit to collect their mate's body.
To bring him home on final time.
His eyes leap to Diana, startled. She doesn't smile, mouth parted in almost fear, hands hovering in the air over Bruce's chest. Blood coats the side of her face, pouring from a cut on her temple, and she favours one side, a hand over the shattered portion of her armour. But the injuries don't compare to the pain in her eyes as they return to Bruce's prone form.
Clark feels a sob wrenched from his throat as he looks down at the body in his arms.
He falls to his knees, bowing over Bruce, listening to that blessed heartbeat.
His heart beats as strongly as ever, stronger, and Clark realises that all the peculiarities he'd noticed can be explained. By the horde of pups surely waiting for them when he returns to the cave, ready for action and probably only still on Earth because the teleporters suffered damage in the explosion.
Can probably be explained by the second, fluttering heartbeat he can hear.
The panicked, confused calls from his coworkers fade away when he tunes into that sound, and he clutches his mate closer, nuzzling into his black hair, the greys that used to populate it a distant memory.
"My mate," he whispers, into those perfect waves. "My beautiful, strong mate."
He strokes his thumb over Bruce's cheek, over where crows feet used to make his handsome smirk that much more suave, and Clark isn't sure when they disappeared, when his knees went back to being weak for his youthful, charming grins instead.
His long lashes flutter over full cheeks, and Clark might stop breathing.
His mate doesn't speak, but one hand comes up, cupping Clark's cheek in return, his glacier-blue eyes wet with relief, and he does his best to maintain that eye contact as he lowers his forehead to rest against the omega's.
