Nashi Dragneel is really interesting. She's often portrayed as having a fiery spirit like her father, Natsu. Then there's Haru Fullbuster, who might inherit his father Gray's ice - making magic and his cool demeanor. Also, the kids of Erza and Jellal can be fascinating characters, as they might inherit a mix of their parents' strong wills and powerful magic.
In Fairy Tail child fanfiction, the character of Nova Heartfilia is quite interesting. As the child of Lucy Heartfilia, she might inherit Lucy's celestial spirit magic and her determination. There's also Kai Redfox, who could be the son of Cana and Elfman. He might have Cana's love for alcohol (in a more innocent, child - like way) and Elfman's strength. Additionally, characters like the descendants of Hibiki and Sherria can add new dynamics to the story, with their potential for unique magical skills.
There might also be new characters created specifically for this fanfiction. These could be allies or enemies of the 'Child of Heaven', adding more complexity to the story. For example, there could be a group that wants to use the 'Child of Heaven' for their own dark purposes, and the Fairy Tail guild has to stop them.
One interesting idea could be exploring the character's new powers as the 'Child of Heaven' in the Fairy Tail universe. Maybe they have unique magic that can change the balance of power among the guilds.
One interesting original character is Lyra. In August Fairy Tail Fanfiction, she is a healer with a special power that can not only heal physical wounds but also emotional traumas. She often gets involved in the most dangerous missions because of her valuable skills. Another character is Finn. He is a thief - turned - good - guy. His skills as a thief are now used to help the Fairy Tail guild in various ways, like stealing important artifacts from the bad guys before they can use them. And there's Seraphina, a powerful sorceress who has a connection to the celestial world and often gives the Fairy Tail mages celestial - related quests.
One plot idea could be a young Erza on a quest to find a magical item that was lost in a hidden part of the Fairy Tail guild. She teams up with other young members and they face various challenges like tricky puzzles and small magical beasts. Along the way, she discovers more about her own magic abilities and the history of the guild.
One of the most interesting characters is Ivan Dreyar from Raven Tail. He has a complex relationship with Laxus from Fairy Tail as his father. There's so much backstory and potential for exploring his motives and his actions within Raven Tail. Another is Obra from Raven Tail. His strange and powerful magic makes him a fascinating character to write about. You can create all kinds of scenarios where his magic is used in different ways.
Gray could be another great pairing. The contrast between his cool - headedness and Lucy's more emotional nature could create interesting storylines. For instance, they might be forced to work together on a mission where their differences initially cause problems but later lead to a stronger bond.
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.
Lucy and Levy are quite popular. Their friendship in the original series can be easily explored in a yuri context. Many fanfictions like to develop their relationship further into something more romantic.
One popular theme is the adventures of the younger characters. For example, the kids of the main Fairy Tail characters might go on a quest to find a lost magical item. Another theme could be friendship among the new generation, just like how the original Fairy Tail members were so close. And there's also the theme of self - discovery, as these young characters figure out their powers and their place in the world of magic.
It's hard to say without actually reading it, but it could feature the classic Fairy Tail characters like Natsu, Lucy, Gray, and Erza. They are the most popular and central to the original series.