Summary: The Healers made a mistake and so James and Lily Potter have a girl instead of a boy. They name their daughter Marigold Dorea Potter and she becomes the Girl-who-lived. Things then turn out differently from there.
Disclaimer for entire fic: I do not own the Harry Potter world. It belongs to J.K. Rowling, Warner Bros., and their affiliates.
James Potter looked down in shock at their new daughter. "But I thought we're having a boy!"
"Well, it's a girl," said Lily, smiling happily. "That does explain why the prophecy Dumbledore told us about said 'he or she'. It means a boy or girl can have the power to defeat evil."
"An evil lord, you mean," James muttered under his breath. Then in a louder and happier tone he continued, "This is wonderful! I have no problems with having a daughter instead of a son."
"What about when she's old enough to date, James?" asked Lily in a teasing tone of voice.
His eyes widened. "She's not dating! I'll keep her locked up in the house until she's thirty!"
"You will not," scolded Lily. "She'll date, but we'll make sure that the boys are decent."
"And I'll be the menacing father that threatens to hurt them if they ever harm my little –"
"Can we come in now?" interrupted Sirius from the doorway. "I want to meet my godson."
"Actually, you have a goddaughter," said James. "We have a girl, not a boy."
Remus, who was standing next to Sirius, looked delighted. "That's great! When you told us about what Dumbledore said, I wondered if maybe Lily or Alice would have a girl. Then the Healers said they were both boys, so I gave up on the idea. Looks like they made a mistake."
"Well, can I see my goddaughter now?" demanded Sirius impatiently.
Lily and James laughed and beckoned Sirius and Remus in. "Where's Peter?" asked Lily.
"His mum said she felt sick and he had to go see if she was fine," responded Remus. "He said he'd return later to meet his honorary godson. He'll be surprised to find it's a goddaughter."
Sirius gently rocked his goddaughter in his arms. "She's pretty. And she looks a lot like you, Lily, but does have something of James too. Like the flecks of hazel in the eyes that otherwise look exactly like yours, Lily. What are you naming her, since you can't use Harry anymore?"
James and Lily exchanged looks. "I don't know," said James slowly. "We never really thought about girls' names, since we thought we'd have a boy. How about Harriet?"
Lily gave him a look. "Absolutely not. I had a great-aunt named that and she was a really horrible person. I wasn't particularly sad when she died a few years ago. The girls in my family usually have flower names, so I think we should do the same for our daughter."
"How about Violet?" suggested Sirius.
James shook his head. "Don't you remember that there's a portrait at Hogwarts named Violet? You know, the Fat Lady's friend? Padfoot, come up with a different flower name."
"Daisy could work," said Remus. "And I don't know anyone, even a portrait, with that name."
Lily hesitated. "Daisy is what my sister wanted to name her son if he'd been born a girl."
"Good point," said Remus, looking thoughtful. "We'll have to come up with something else."
"How about Marigold?" suggested James. "It's pretty name, and so's the flower."
"Oh, that's perfect!" exclaimed Lily. "There's a book by Muggle author L.M. Montgomery called Magic for Marigold. Her books were some of my favorites when I was a girl."
"So I assume that this book was about a girl named Marigold?" asked Remus.
"Yes, and she was a very lovely character," replied Lily. "She had a wonderful imagination, even creating an imaginary friend named Sylvia, and did all sorts of lovely things."
"All right then, we'll name our daughter Marigold," declared James. "Now for a middle name."
"How about Dorea, after your mother?" asked Lily.
"I like it, but what about those books you liked when you were younger? Did that author ever write a book about a red-haired girl, since our daughter has red hair?"
"There was one. Well, it was a series, about an orphan girl named Anne Shirley, later Blythe when she married. An elderly brother and sister wanted to adopt a boy, mainly to help around their farm, but the orphanage sent a girl by mistake and they ended up keeping her. Anne didn't like her red hair much, or her name, and insisted that her name be spelled with an 'e' since she couldn't change it to Cordelia, which she preferred. She also had an imagination."
"I don't mind the imagination part, but I don't want our daughter hating her middle name or her red hair," said James. "So unless you really want it to be Anne, let's stick with Dorea."
"All right," said Lily. "Dorea is a lovely name, and I love your mother."
"That's settled then," said Remus, taking Marigold from Sirius. "She's Marigold Dorea Potter."
An hour later, Peter turned up, saying that his mother was feeling better. He was surprised to discover that he was honorary godfather (Sirius was the true godfather and Remus had been jokingly proclaimed 'god-wolf' and not jokingly made the other honorary godfather) to a girl. Once he'd gotten over his surprise, he complimented the name and held Marigold until she started crying loudly a minute later. Lily took her daughter back, saying that she likely wanted to be fed, and amazed everyone by calming down the second she was in her mother's arms.
Incidents like this happened throughout the next year, with Marigold putting up a fuss whenever Peter tried to hold her. When she was presented with stuffed animals that were the Animagus and wolf forms of the Marauders, she tried to throw the rat one in the fire. She did this unsuccessfully several times. However, she loved the other three, though she loved the stuffed stag the best and seemed to prefer the stuffed dog to the stuffed wolf. Sirius took this as a sign that Marigold loved him more than she did Remus, who just shrugged it off.
Lily, worried about what might happen, put an Undetectable Extension Charm and several protection spells on a box. She then filled it with a number of books that she had enjoyed reading as a girl, some of her and James's Hogwarts things, and a few letters from her and the Marauders, minus Peter (who wasn't much of a letter-writer.) Later she added some pictures, a few pieces of jewelry a beautiful doll with a bag of clothes and accessories, and a few toys. In case something was to happen to Lily and James, Marigold was to get the box.
Marigold's happy little world ended on Halloween of 1981. The Potters' Secret-Keeper betrayed the location to Voldemort, who then set out to kill them. While he believed that no mere girl could defeat him (though two of his Death Eaters were female and quite formidable, especially Bellatrix Lestrange), he wanted to be on the safe side. Also, he couldn't exactly target the other child that fit the prophecy, Neville Longbottom, as he didn't know the location yet.
Voldemort got the shock of his life after he killed James and then forced to kill Lily when she refused to step aside and give up her daughter. The Killing Curse rebounded off Marigold, leaving only a lightning-shaped scar on her forehead, and struck Voldemort. Greatly weakened and without a corporeal body, he was forced to flee the mostly ruined house.
On Professor Dumbledore's insistence, Marigold and the special box her mother had packed for her were sent to the Dursleys, her only living close relatives. Professor McGonagall reluctantly agreed with the decision and helped put some spells on the contents of the box so that Muggles wouldn't notice the magical items or forbid Marigold from keeping them. After that, the two professors and Hagrid left a letter explaining things, wished the child well, and left.