A powerful dark guild could be the main antagonists. They might be jealous of the combined power of Natsu and Mirajane's family and seek to destroy them to gain more power in the magical world.
An old enemy of Fairy Tail from the past could resurface. This enemy has a vendetta against the guild and specifically targets Natsu and Mirajane's family because they represent the new hope and strength of the guild. Maybe this enemy has some dark magic that can disrupt the family's harmony and they have to fight against it.
A rogue wizard who is obsessed with collecting rare magical powers could be the antagonist. He discovers that Natsu and Mirajane's family has some unique magical traits that he wants to steal for his own gain. This wizard uses various devious means to try and break into their family life and take what he wants.
The main characters are clearly Natsu and Mirajane. Natsu is often portrayed as a hot - headed and powerful wizard. Mirajane, on the other hand, is a strong and beautiful character with her own set of skills and a complex personality.
A group of dark fairies could be the main antagonists. They might be jealous of Natsu's power or his unique status as a fairy and try to undermine him at every turn.
It could involve their daily life together. Maybe Natsu is still going on adventures, and Mirajane is supporting him while also taking care of their home. There could be cute moments like Natsu bringing back strange things from his quests and Mirajane just shaking her head with a smile.
There could be a powerful demon from another dimension. As Natsu becomes an adult, his magic signature becomes stronger, which attracts this demon. The demon wants to possess Natsu's power for itself and will stop at nothing to achieve this goal. It would be a formidable foe, with abilities that Natsu has never faced before.
Maybe an old enemy of Natsu's resurfaces. This enemy is jealous of Natsu's newfound happiness with Cana and tries to take Cana hostage to force Natsu to fight him. This would create a major conflict in the fanfiction.
It could be another guild that sees Natsu as a threat. They might find out about Lucy kidnapping Natsu and try to use the situation to their advantage. For example, they could try to steal Natsu's magic or get information out of him while he's in a vulnerable state. Another possible antagonist could be a character who has a grudge against Lucy. They might try to stop her from getting away with the kidnapping or use it as a way to get revenge on her.
Perhaps a rogue ninja group that has a vendetta against Anko. When she forms this new family with Naruto, they target the whole family unit to get back at her. They might see Naruto as a way to get to Anko or vice versa, and constantly attack and create chaos for them.
Obviously, Natsu and Mirajane are the main characters. Natsu, with his red - haired, energetic self, brings his unique charm and his powerful fire magic. Mirajane, with her white hair and her ability to transform into different forms, adds her own flavor to the story. And the baby, which is new to their lives, serves as a catalyst for new adventures and character development.
The main characters are obviously Natsu and Mirajane. Natsu, who has a dark side in this fanfiction concept, is known for his fiery magic and impulsive nature. Mirajane, on the other hand, is a powerful mage with her demon form capabilities.
It could be a powerful dark wizard who has a grudge against Fairy Tail. This wizard might see Natsu and Mirajane's joining as an opportunity to strike at the guild when it's in a state of flux. He could be someone who was once defeated by Fairy Tail in the past and has spent years building up his power for revenge.