There could also be some characters from their past that come back into the picture. Maybe an old acquaintance of Gray's who has some knowledge about his family history related to pregnancy and child - rearing. This person could offer some unique insights. And on Lucy's side, perhaps a relative she hasn't seen in a while shows up to help or cause some drama during this special time.
Another character who could interact with them is their guild master. He could be concerned about how this pregnancy will affect their ability to take on jobs and contribute to the guild. He might offer some sort of support or special arrangements for them during Lucy's pregnancy. And of course, there could be some side characters from their world who interact with them in different ways, like the local merchants who provide baby supplies.
Natsu would be a very likely character. Since he and Lucy are close friends in the original story, if Lucy travels back in time, Natsu might be one of the first people she seeks out. They could have adventures together in the past, with Natsu being his usual impulsive self and Lucy having to keep him in check with her knowledge of the future.
Erza could be a great supporting character. She might offer her strength and protection to Lucy during the pregnancy. Natsu could also be there, being his usual boisterous self but also showing a caring side towards the expecting couple.
Gray's character development could be quite significant. He might start to show his softer side more often. Usually, he's all about the ice - cold exterior, but now he has to think about a family. He could become more of a family man, putting Lucy and the baby's needs above his own. For Lucy, she'd grow in confidence. She'd have to be strong for the baby, and this would make her more assertive in the guild. She might also start to take on different roles, like teaching the younger members about family values that she's learning herself.
The plot could involve Gray and Lucy dealing with the challenges of pregnancy. For example, Lucy might have a difficult pregnancy with some health scares. Gray would be by her side, using his ice magic to keep her cool during hot flashes. Their friends might be concerned and offer different kinds of support. In the end, they welcome a healthy baby into the world.
Gray and Lucy's pregnancy could be a beautiful journey. One day, Lucy discovers she's pregnant. Gray is initially shocked but then filled with joy. They start preparing a small nursery in their home. Gray paints the walls blue while Lucy arranges the baby furniture. Their friends from Fairy Tail are excited too, constantly bringing gifts and offering help.
Obviously, Naruto and Lucy would be the main characters. Naruto as the son of Lucy would be at the center of the story. There might also be other Naruto -verse characters like Sasuke, Sakura, and Kakashi. They would interact with Naruto in the context of his new identity as Lucy's son.
Gray's protective side comes out when he's with Lucy. Since Lucy can be a bit naive at times, Gray feels the need to safeguard her. At the same time, Lucy's sassiness can keep Gray on his toes. She's not afraid to call him out when he's being too cold or distant, which in turn helps Gray to open up more.
Natsu could be a character who betrays Lucy in some fanfictions. Maybe he gets influenced by some dark magic that makes him turn on her. Gray is also a possibility. He might be jealous of Lucy's growing power or popularity in the guild and decides to betray her for his own gain.
Maybe the story could focus on Lucy's journey during the pregnancy. She could be dealing with morning sickness and hormonal changes while Gray is away on some important mission. This could lead to some misunderstandings between them, but in the end, they come closer together when Gray realizes how much Lucy needs him.
One idea could be that Lucy discovers she's pregnant during a difficult time in the magical world. Gray is initially shocked but then becomes very protective. They might face challenges from their guild mates who are worried about how this will affect their ability to go on missions.