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.
I'm not sure specifically as it could cover a wide range of things. It might be a fan - made story in the 'Fairy Tail' universe that involves some sort of planetary anchor concept. Maybe it's about a magical item or power that acts as an anchor for a planet in the 'Fairy Tail' world.
You could try fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of 'Fairy Tail' fanfictions and there's a good chance you'll find the 'fairy tail planetary anchor' one there.
One idea could be that the planetary anchor is a powerful ancient relic that gets discovered in the Fairy Tail world. It has the power to stabilize the magic energy of the planet, but an evil force wants to use it to drain all the magic. The Fairy Tail guild has to protect it and figure out how to use it for good.
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.
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.
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.
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.
In a fanfiction, the Planettary Anchor could be a catalyst for character growth. For the younger members of Fairy Tail, it could be a test of their courage and magical abilities. They might be forced to step up and learn new skills in order to deal with the situations that the anchor creates. At the same time, the more experienced members would have to reevaluate their strategies and leadership as the anchor brings in new and unexpected challenges. This would lead to a shift in the overall dynamics of the guild as everyone adapts to the changes brought about by the Planettary Anchor.
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 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.