With their daughter, there would be a sense of unconditional love that's different from their romantic love. They would love her no matter what mistakes she makes. In their relationship with each other, they have to work through issues and misunderstandings. But with their daughter, they would be more forgiving and patient from the start.
Well, their relationship with their daughter would be more parental. They would be protective and nurturing, which is different from their more equal, romantic relationship with each other. For instance, Castle might be the fun - loving dad who tells bedtime stories, while Beckett would be the strict but fair mom who makes sure she does her homework.
In a fanfic where they have a baby, their relationship would shift. They would now be partners not only in crime - solving but also in parenting. Castle would likely become more domesticated, helping with diaper changes and midnight feedings. Beckett would have to learn to let Castle in more on the parenting front, and they would communicate more about family - related matters. This new aspect of their relationship would add a different layer of complexity and closeness to their bond.
With Ron, their relationship could be a bit rocky at the start. Ron is used to the normal wizarding world and Harry's unique heritage as the daughter of Artemis might seem strange to him. However, as they go on adventures together, he would come to respect her abilities. Her relationship with Dumbledore would be one of mutual respect. Dumbledore, being the wise old wizard, would recognize the potential in Harry's connection to Artemis and guide her in using it for good. He might also be intrigued by this new aspect of magic in the wizarding world.
Harry would likely be a loving and doting father. He'd teach her everything he knows about magic, from basic spells to advanced defensive magic. However, he might also be a bit overprotective because of his own past traumas. He would have a close relationship with her, sharing stories of his adventures at Hogwarts and during the fight against Voldemort.
Bellatrix was extremely cruel and devoted to the Dark Lord. Her daughter could be the opposite. Maybe she has a kind heart and is repulsed by the dark magic her mother practiced. She could be more empathetic towards others, especially those who were victims of the Death Eaters.
At first, their relationship could be rocky. Beetlejuice is a chaotic character, and the daughter might be a bit wary of him. But as time goes on, they could start to understand each other better. Maybe she sees the good in him that others don't.
Her relationship with Ron and Hermione would be different. They might be in awe of her divine heritage at first. Ron could be a bit more protective, seeing her as something special. Hermione would be curious and eager to study about Celestia because of her connection to Harry. And with Draco, he might be more envious or try to use her secret against her.
Perhaps their daughter has a special talent that gets her involved in a dangerous situation. She could be a prodigy in computer hacking, and while trying to help her parents on a case, she uncovers a secret that puts her life in danger. Castle and Beckett then have to race against time to protect her while also solving the case.
He might get jealous right away. He could storm off and then later come back to confront Beckett about it.
They'll become even closer. The knowledge of a new life will bring them together on a deeper level. There'll be more tenderness between them.
If Erza was Naruto's daughter in fanfiction, her relationship with Sasuke could be interesting. Maybe Sasuke would see her as a reminder of Naruto and be both protective and a bit standoffish at first. With Sakura, she could be like a little sister figure, and Sakura would help her with her medical ninjutsu skills if she wanted to learn. And with Kakashi, he could be her mentor, just like he was to Naruto, guiding her in the ways of the ninja.