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.
A possible plot is that the future child discovers a hidden prophecy about their family. This prophecy could say that their actions will determine the future of the entire magical world. So, they embark on a journey to understand their place in this prophecy. Along the way, they meet different characters from the Fairy Tail universe, both old and new, and learn about the true meaning of family and friendship from their parents' past experiences.