Perhaps the baby girl is left at the fire station. The fireman takes an immediate liking to her and starts the adoption process. The story could then follow how he adjusts his life for her. He might have to learn about baby care, deal with his own emotional baggage, and also face the reactions of his colleagues and family. For example, some of his colleagues might be really supportive, while others might be skeptical at first. And his family could have mixed feelings too, but over time, they all come to love the baby girl.
In some plotlines, the fireman could be a widower or someone who has always wanted a family but never had the chance. When he adopts the baby girl, it's like a new beginning for him. He may struggle with financial issues at first, trying to provide for the baby girl on his salary. The story could also involve the baby girl having some sort of special talent or a mysterious past that gradually unfolds as she grows up, adding an extra layer of intrigue to the story.
One possible plotline could be that the fireman discovers the baby girl during a rescue mission. He then decides to adopt her out of a sense of protectiveness. As she grows up, he has to deal with the difficulties of being a single father while also being a fireman.
There are many fanfiction platforms out there. Some popular ones are Wattpad as well. However, I haven't personally read a 'fireman adopts baby girl' fanfiction, but often these types of stories might explore themes like the fireman's newfound family, the challenges and joys of raising a child in an unexpected situation, and how the baby girl changes his life for the better. So when you search, look for stories with high ratings and positive reviews to find good ones.
Perhaps in the fanfiction, Harry adopts Tom Riddle when he's still a young boy at the orphanage. Harry then uses his own experiences of growing up without parents to bond with Tom. He teaches Tom about magic in a different way than Dumbledore did, focusing on the positive aspects of magic for good. This could lead to Tom not becoming the evil Voldemort we know.
One possible plotline could be that the female Harry Potter stumbles upon Naruto's world through some sort of magical accident. She then realizes Naruto's situation and decides to take him under her wing, teaching him about magic in the process. Another could be that she is sent on a mission to Naruto's world for some magical reason and sees Naruto's potential and adopts him as her protege to help him grow stronger both in ninja skills and, with her help, in magic as well.
One possible plotline could be Luna seeing Twilight's potential and taking her under her wing to teach her some ancient and forgotten magic. This would lead to Twilight having unique abilities and a special bond with Luna.
Maybe there could be a conflict where other characters in the Sonic universe don't trust Shadow with baby Sonic. They might think Shadow has ulterior motives, and Shadow would then have to prove that he just wants to give Sonic a good upbringing. This could lead to a lot of drama and interesting interactions between the characters.
One possible plotline could be Lucifer realizing that Trixie needs a stable family figure and decides to step in as her adoptive father. He then has to deal with the challenges of raising a human child while also being the Devil.
One possible plotline could be Naruto's first attempts at using chakra as a baby. It would be really cute to see him unknowingly channel his chakra while playing with a toy, and then get a big surprised look on his face when something unexpected happens.
Well, a plotline might involve the cultural differences between Brienne and Tormund coming into play when raising their baby. Tormund's wildling ways and Brienne's more chivalrous background could lead to some interesting parenting moments. Also, there could be a plot where the baby is seen as a symbol of unity between different factions, and they have to deal with the pressure of that. It could also show how they adapt to their new roles as parents while still being true to their own characters.
Perhaps a plotline where the baby grows up in the care of Makino while Shanks is away at sea. The story could show the baby's longing for its father and how Makino tries to keep the memory of Shanks alive for the child. As the baby gets older, it might start to show an interest in following in Shanks' footsteps, which could create some interesting family dynamics and internal conflicts for both Makino and the child.
One possible plotline could be the challenges they face in raising the baby. Maybe there are political or family issues that they need to deal with while protecting their child.