As the title suggests, Steve and Bucky sleep together and Bucky ends up pregnant from it left behind in Wakanda.
Very simple concept, lots of ideas can be spun around this one. I have seen variations of the idea made. An author named 'Jibbly' made a story called 'Counting Sheep', (which I think is a great story and definitely deserves some lovin', hell, all of Jibbly's stories could use some love, I know my stupid ass should comment more and love people but... go love Jibbly~... please, super desperate and needy here for some fine ass Stucky stories, lol) in which Bucky gets pregnant before going under ice in Wakanda, but instead of Steve leaving and not knowing, he stays with Bucky, they forget about the cryogenic chamber for the baby, they have their baby and it's super cute. Lovely story.
Where this concept would differ is that maybe they sleep together before Bucky goes under ice or after when Steve visits, but the whole point would be that Steve leaves to hero it up without knowing he got Bucky pregnant, without either knowing initially. Bucky could still go under ice if it's too early for them to know, like... all he got is Steve's sperm in him... or just... you know~ there are several ways to do this though.
With Alpha/Beta/Omega Dynamics, they could have been careless, maybe Bucky wasn't in heat or Hydra supposedly sterilized him and either his serum fixed it or Steve's serum did. Or maybe it just wasn't something they thought about in the heat of the moment. Maybe they were mates who had been apart too long and had to renew their bond in a sudden shocked heat and rut after all the fighting. Maybe Steve's just stupid, or he breaks the condom. Maybe they weren't taught proper sex ed and got really damn lucky when they were young or Steve being pre-serum meant he wasn't fertile and they never got the chance to test it during the war, the list goes on.
Another way to do it, and I honestly still have no idea why no one ever thinks of this as it can be a concept explored all on its own, is as simple as super sperm. Or more specifically, Steve's super serum sperm. I've always always loved the idea of him accidentally getting someone pregnant who shouldn't get pregnant this way. Because the serum is responding to Steve's drive to 'reproduce' and doing what it can to change or 'heal' the individual he is 'donating' it to. Steve's serum is supposed to be perfect, so it would be able to fight off any changes in him, but I don't think it's too far stretched it would try to promote changes in others. It's likely it might take a few 'tries' or maybe just a lot of super sperm to 'make the change', but if they were fucking during the war in this version, bucky could already be lined up and ready for it without even knowing, or they just fuck A LOT and shit happens. Or maybe Steve's serum really is that perfect and it only takes one goddamn time. I could imagine Bucky being a little grumpy about that one. I could also imagine the serum also 'changing' someone to be specifically 'for' Steve, so maybe Hydra even noticed and tried to get Bucky pregnant and failed, there's a lot of opportunities for creativity here.
Another way I have seen done a few times, but maybe not as frequently as I would expect, is that Hydra experiments on Bucky and he is able to carry children because of it. Now, considering that this would dwell on the concept of neither knowing to begin with, I'm going to say that this is a fairly recent addition from Hydra that was never tested, maybe because it was just too recent for them to get a chance to test or they only wanted to test it once they found Cap again, desperately wanting that super soldier sperm~... I should stop... but I probably won't? Did I mention I'm trash. Back to Bucky. Maybe Bucky just got too damn aggressive anytime they tried to test it and they were afraid to do anything else. (Anything for that happy ending and no successful trash parties because ya boy has been though ENOUGH. Clearly, I am doing everything I can to avoid HTP here. He needs that happy ending... I NEED THAT HAPPY ENDING.) It could also be a side effect of his botched version of the serum that Hydra maybe played a little too much with, or maybe Zola wanted to make Bucky the perfect counterpart for Schmidt to bear mini supersoldiers of the next generation for Hydra, but the idea never came to fruition when Schmidt 'died' and Bucky escaped the first time (and of course we all know Bucky's a much better match for Stevie~).
Bucky could even just be trans or born intersex, and then related above issues, infertile because of Hydra or other, etc. Stevie fixes it with his fuckin' super sperm... that's an option cause the dark lord knows we just need so much more of Bucky with a pussy out there... Like goddamn. I have the worst craving for boypussy Bucky...
I mentioned I'm garbage, right?
Once we establish that our Bucky boo is pregnant and Steve is off being a hero, there are even more ideas to explore. Steve could come back and find out while Bucky is still pregnant, Bucky could reach out to him through T'Challa, or maybe he doesn't want to bother Steve so Steve doesn't come back until the kid is born and could be a little hurt by Bucky never telling him, or just in shock. It all depends. It can be a strict Canon divergence with no Infinity Wars or Endgame, the kid even being a little older when Steve finally does find out (because he knew Bucky had a kid but maybe not that he gave birth to it or that it was because of Steve) or if you wanna make it longer, it can go even deeper into it with Steve losing his family, missing them, maybe not even getting to know his child all that much before the snap after having just found out (maybe even prompting arguing with Bucky and possible babymaking angry sex~), and then fighting to get them both back.
There are so many ways it could be done. The way I explored this idea had Bucky initially 'stealing' a fragmented cosmic cube from Hydra, which could also be responsible for his baby carrying capabilities or even heal his infertility if he's sterilized from A/B/O, from being intersex, or whatever. This cosmic cube then 'disappears', but reappears when Bucky gets pregnant in his and Steve's daughter, Kobik. This is the SuperSoldierFamily route.
If you don't know who Kobik is, you can go ahead and look her up for better detail, just seach 'kobik 616' and click the link for the wiki, but she is basically, in the comics, a cosmic cube that took sentience in the form of a little girl because she felt naive to the world, and a childish form suited her best. She is super adorable, super innocent, super powerful though with limited control of her powers, and Bucky takes care of her like this big ol' sweetheart mama bear and it's just, ugh... it's heart melting~...
The way cosmic cubes also work is that they are kinda like complex 'wishing' cubes? Sorta, they are reality warping to an extreme, very powerful, and very dangerous in the wrong hands if those hands know how to use them, and that's the kicker. Cosmic cubes are also multiversal, nearly omniversal if they aren't somehow, so they are more powerful than the infinity gauntlet with all 6 stones, which only hold power on a universal level (as they are linked to the universe they are from), i don't know why they didn't just find one a fuckin' these to fix everything, cause... you know... but not the point.
Kobik would not be happy about losing her mother (Bucky), probably still be a baby when the snap happens, and would either be able to cock block Thanos completely, immediately reverse it, or maybe even change his mind and have him surrender and ask to be killed. Like, we get it, the bitch just tryin' to get laid, but damn. For those that don't know, Thanos in the comics is actually in love with the universal Entity known as Death. All souls belong to Death, and as it is in her nature to be... Death itself, he actually kills people to please her, because he's courting her, because he's head over heels in love with her and just wants her to notice him... you know. It's actually kinda sad and sweet, but like... the boy is DEVOTED.
Anyway, Kobik could even just bring back Bucky after the snap (cause she's a baby, what kind of control could she have apart from wanting her parents?). Steve hears her crying and runs to see him rematerialize at his little hut where she is, maybe Steve still doesn't know about her, and Bucky's freaked out but they both hang onto each other when everyone's still just registering what happened and trying to figure it out.
SO MANY IDEAS~!!
Anyway, I really hope someone likes this idea enough to pick it up, and I would more than be open to discussing even more. If I can get to it and write my own fic, I'm definitely going the Kobik route but I just don't know if or when that will be, and the Kobik ideas are still open to anyone who wants to use them, I don't mind at all, we definitely need more SuperSoldierFamily. If you're interested, please let me know and keep me posted, I would love to see it.
Satan knows we could always use more bearded Nomad Steve railing Bucky in T'Challa's broom closets~.
WORKS FROM OTHER AUTHORS~!!!:
Specifically the prompt<3~
More Loosely inspired by (BUT STILL AMAZING!!!) ;)~