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.
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.
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.
Maybe a rival guild that is jealous of Fairy Tail's new addition. They might think that having a baby with such strong magical potential in Fairy Tail gives them an unfair advantage. So they try to kidnap the baby to either keep it for themselves or use it to bargain with Fairy Tail.
One idea could be that Natsu and Mirajane start a family in a peaceful post - guild - war era. Their kids inherit unique magical abilities from both of them and they have to train and guide their children through the magical world's challenges.
The dark guilds would be great antagonists. They might see Natsu's new leadership as a threat and try to undermine his guild. They could send their best mages to disrupt the guild's operations and steal important magical artifacts.
One possible antagonist could be a dark wizard from a rival guild who is jealous of Natsu's status as an alpha in Fairy Tail. This wizard might try to undermine Natsu's leadership and the guild's reputation.
The Fairy Tail guild members who supported Natsu's exile could be antagonists. They might believe in false accusations against him or have their own motives for wanting him out of the guild, like jealousy of his power or popularity.