They could be a key to a hidden treasure. In the Fairy Tail world, there could be a legend that says the scarlet lilies hold the key to a great treasure. So the characters go on a hunt for these lilies. Another way is as a power source. Maybe some dark wizard wants to use the energy of the scarlet lilies for evil, and the Fairy Tail guild has to stop them.
One common theme is friendship, just like in the original Fairy Tail. Scarlet lilies might be used as a symbol of a deep and unbreakable bond between characters. Another theme could be magic. Maybe the scarlet lilies have some sort of magical power in the fanfiction, like being able to heal or enhance a wizard's abilities. Adventure is also likely. The characters might go on a quest related to the scarlet lilies, perhaps to find the rarest of them or to protect them from an evil force.
Mermaids could be introduced as a new species that live in the deep, unexplored parts of the ocean in the Fairy Tail world. They could have their own magic system related to water and sea life. For example, they might be able to communicate with sea creatures and use them in battles.
One way could be through a crossover spell. Maybe a powerful mage in Fairy Tail accidentally creates a portal that brings Vegito into their world. Then Vegito's incredible power would be a huge shock to the Fairy Tail guild, and he could either be an ally or a misunderstood foe at first.
One way is to create characters in the real world who possess the magic abilities seen in 'Fairy Tail'. For example, a character could have Natsu's fire - breathing magic and use it to fight crime in a modern city. Another approach is to set up guild - like organizations in the real world where people with different 'Fairy Tail' - inspired powers come together to complete missions or help the community. Also, you could have real - world locations become the settings for 'Fairy Tail' - style adventures, like an ancient forest being the site of a quest for a magical artifact.
They could be incorporated as a new threat that the guild has to face. Maybe a dark wizard has unleashed a horde of boggarts on Magnolia, and the Fairy Tail members have to use their unique magic to defeat them.
Well, it could be incorporated as a new form of magic. A character in Fairy Tail might discover an ancient spellbook that has a similar power to Kamui. This discovery could change the power dynamics in the guild and lead to new adventures.
In a Fairy Tail fanfiction, the Wood Release could be a lost magic. A character, perhaps a descendant of a long - lost civilization, discovers they have the ability to use Wood Release. They could use it to protect their guild or to fight against dark mages. The Wood Release could be used to create barriers made of thick, impenetrable wood or to trap enemies in a forest of magically - grown trees.
It could be incorporated as a McGuffin. For example, the characters hear rumors about the planetary anchor and its great power. They then go on a quest to find it. Along the way, they encounter various challenges and enemies related to those who also seek the anchor.
One way could be through a magical portal. Maybe a powerful mage in Fairy Tail accidentally creates a portal that links to the Naruto universe and Madara somehow gets sucked through it. Then he could use his powerful jutsu in the Fairy Tail world, creating all sorts of chaos and new storylines.
Well, it could be that a character from Fairy Tail somehow travels to the Naruto world and acquires the Rinnegan. Then, when they return to their own world, they have to adapt it to the magic - based system. They might find that the Rinnegan's abilities interact strangely with their own magic, creating new and unexpected effects.