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.
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.
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.
Some fanfictions feature ancient evils that were sealed away long ago but are now starting to break free. These could be powerful demons or corrupted spirits. They see Lucy and Natsu as threats to their plans of world domination, so they target them. The antagonists in these cases are not only physically powerful but also very cunning, constantly devising new ways to outwit our heroes.
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.
Well, there could be such a fanfiction out there. However, it really depends on the creativity and interests of the fanfiction writers in the relevant fandom. Some might find this concept interesting to write about, while others may not. It also might not be very common as it involves a rather specific and perhaps controversial plot point in the context of the characters.
In a great Mirajane and Natsu fanfiction, the author should be able to blend the magical elements of the Fairy Tail world seamlessly. For example, if they're facing a powerful enemy, the way they use their magic should be in line with the original series. Also, the fanfiction should explore the emotions of the characters deeply, whether it's Natsu's determination or Mirajane's compassion, making the readers really connect with them.
Some Tumblr blogs are also dedicated to Fairy Tail fanfictions. Browse through the Fairy Tail tags on Tumblr and look for posts related to Natsu and Mirajane's relationship. There's a chance you'll stumble upon some great married fanfiction stories about them.
One common theme is friendship. Since they are both part of the Fairy Tail guild, many fanfictions explore the deepening of their friendship over various adventures. For example, they might go on a mission together and learn more about each other's strengths and weaknesses, which in turn strengthens their bond as friends.
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.