Well, you could write a fanfiction where in the 'Watch the Future' scenario, the future shows that the magic in the world is starting to disappear. The Fairy Tail members then have to embark on a quest to find out why and how to stop it. They might discover an ancient evil that was sealed away long ago and is now causing this as a side - effect of trying to break free. Also, a story about how the future reveals a betrayal within the guild by one of their own could be really interesting.
One idea could be exploring how the next generation of Fairy Tail wizards handle a new dark guild threat. They might have to combine the old and new magic techniques they've learned.
One interesting idea could be how the characters use their knowledge of the future to prevent disasters. For example, if Natsu knew that a powerful dark guild was going to attack Magnolia in the future, he might train harder or seek out new powers to stop it. Another idea could be the moral dilemmas that come with seeing the future. If Lucy saw that a friend would die in a future battle, would she try to change it even if it meant disrupting the natural order? And perhaps some fanfictions explore how seeing the future affects the relationships between the characters. Maybe Gray, who has always been a bit reserved, becomes more open and protective of his friends when he realizes what could happen to them in the future.
One idea could be Future Rogue trying to change the course of a major event in Fairy Tail's future. For example, preventing a powerful enemy from attacking the guild. He could use his knowledge of the future to his advantage.
One idea could be the children of the main Fairy Tail characters going on a quest to find a lost magical artifact that was once safeguarded by the guild. This would test their skills and teamwork, just like their parents before them.
Perhaps a plot where Future Natsu has lost his memory of the future but still has his enhanced powers. The Fairy Tail guild has to figure out who he is and why he's there while he tries to regain his memories. This could lead to many comical and heartwarming moments as he bonds with the guild members all over again.
Perhaps the future child is born with a special mission. In the fanfiction, it could be that the child has to reunite different factions of the Fairy Tail guild that have split apart in the future. This would involve the child using their knowledge of their parents' friendship and the power of Fairy Tail's bonds. Also, the future child might face an enemy that is related to an old foe of Fairy Tail, and they need to draw on their family's legacy to defeat it.
You could do a time - travel fanfiction. Have a character go back in time to when the Fairy Tail guild was first founded and see how they interact with the founders. Or, write a story where the characters swap powers. So, Erza would have Lucy's celestial magic and Lucy would have to use Erza's requip magic. This would lead to a lot of comical and also challenging situations for them. Also, you could write a fanfic about the guild taking on a mission in a completely new and strange land, filled with magic they've never seen before.
One idea could be exploring an alternate universe where the main characters have different magic abilities. For example, Natsu could have ice - based magic instead of fire. Another idea is to focus on the backstories of the minor characters in more depth, like how they came to join Fairy Tail. A third idea would be to create a time - travel fanfiction where characters go back in time to change a significant event in the Fairy Tail world.
One idea could be exploring an alternate universe where the main characters have different magic powers. For example, Natsu could have ice - based magic instead of fire. Another idea is to focus on a backstory for a minor character, like how they came to join the guild. Maybe a story about a character's journey to find a long - lost magical artifact that has a connection to Fairy Tail's history.
You could write a Fairy Tail fanfiction centered around a character's growth. For instance, take Wendy. You could write about her journey to master new and powerful dragon - slayer magic, facing challenges that force her to grow not only in power but also in confidence and leadership skills. It could involve her going on a solo adventure to a far - off land to learn from a reclusive dragon.