A plot could revolve around Future Rogue attempting to redeem himself. He realizes that the path he was on in the future was wrong and tries to make amends in the present. He could start by helping the Fairy Tail members in small ways and eventually prove his loyalty and change of heart.
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 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.
Maybe a plot where Yukino gets kidnapped by an enemy, and Rogue has to go through a series of trials to rescue her. During this, he discovers new aspects of his own power and his feelings for Yukino. It could involve a lot of action scenes as Rogue battles the kidnappers and their minions. Also, there could be some emotional moments when he finally reaches Yukino and they share a tender moment.
One idea could be that Lucy and Rogue are brought together by a powerful magic artifact. They have to team up to protect it from an evil force that wants to use it for destruction. Along the way, they start to understand each other's pasts and form a deep bond.
One idea could be a story where Sting and Rogue have to go on a special mission together in a new, uncharted land. They face various magical creatures and have to learn to trust each other even more. Another could be a fanfic set in an alternate universe where they are not part of Fairy Tail, but are instead traveling rogues who cross paths with characters from Fairy Tail in unexpected ways.
Perhaps there is a prophecy that only Rogue and Kagura together can fulfill. It could be about restoring a long - lost magical artifact that has the power to bring peace between their two warring lands. Along the way, they face opposition from those who want to use the artifact for evil. As they work towards the goal, they learn more about each other's cultures and values, and their bond grows stronger.
One idea could be a story where a rogue mage from a different guild infiltrates the Fairy Tail guild and causes chaos among the members. They could be stealing important artifacts or trying to disrupt the relationships within the guild.
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.
One idea could be a time - travel plot where Lucy, Rogue, and Sting are sent back in time to the founding of Fairy Tail. They have to navigate the early days of the guild while also trying to find a way back to their own time. Another could be a magic - swapping scenario, where they accidentally exchange their magical abilities and have to learn to use each other's magic to fight a new, powerful enemy.
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.