One common theme is redemption. Since Pain is often seen as a villain in his original context, many fanfictions explore his path to redemption within the Fairy Tail world.
Power struggles are also a big theme. Pain has powerful abilities and when he enters the Fairy Tail world, there are often clashes with other powerful mages in the guild over power, territory or ideology. This creates a lot of exciting storylines. For example, how he might compete with Natsu's fire - based powers or Erza's versatile combat skills. Another aspect is the theme of acceptance. How the Fairy Tail members come to accept or reject Pain in their midst, considering his past actions and his potential for change.
One common theme is redemption. Since Silver has a complex past, many fanfictions explore his journey towards redeeming himself. Another theme is friendship, as he is part of the Fairy Tail guild, and his relationships with other members are often explored.
Another theme is power. Since both Touka and Fairy Tail characters have their own powers, fanfictions often explore how Touka's power can interact with or be enhanced by the magic in Fairy Tail. It could be that her unique abilities are combined with Fairy Tail magic to defeat a powerful enemy.
In many 'Fairy Tail End is Natsu' fanfictions, the theme of redemption can be found. Natsu might have to make up for past mistakes or face the consequences of his actions in a more profound way as the end approaches. Another common theme is the exploration of his dragon - slayer heritage. How his connection to dragons plays a role in the final events of the story is often explored. And of course, the theme of unity within Fairy Tail. Natsu is usually at the center of bringing everyone together to face the final threat or to celebrate the end of a long journey.
Often, there is the theme of loyalty. Even in a slave - like situation, Natsu's loyalty to his friends or certain ideals might be tested or emphasized. Also, growth is a theme. He may grow as a character through the challenges he faces in this different status compared to his normal role in the Fairy Tail world.
Common themes in fanfictions about Natsu and Mira from Fairy Tail include growth. Both characters can grow as individuals through their interactions. Natsu might become more mature in how he approaches problems, and Mira could become more confident in her abilities. The theme of family is also there, as the Fairy Tail guild is like a family. Natsu and Mira's relationship within this family - like environment can be explored, such as how they support each other during tough times in the guild or when dealing with personal issues.
The concept of time distortion is a major theme. In chaos time, time may not flow as it should. This can lead to characters experiencing events out of order or having to deal with the consequences of actions that haven't happened yet in the normal timeline. Another common theme is the struggle for survival. The chaos makes the world a more dangerous place, and characters have to fight to stay alive and protect their loved ones.
One common theme is power growth. Usually, the water devil slayer starts with a certain level of power and then goes through various challenges and training to become stronger. Another theme is friendship. Since it's set in the Fairy Tail universe, the slayer often forms strong friendships with other guild members. And of course, there's the theme of battling against dark forces. The water devil slayer uses their powers to fight off villains and protect the guild and the innocent.
One common theme is the 'forbidden love' aspect. Since they are comrades in the guild, some fanfictions play on the idea that their love is something they have to keep secret. Another theme is 'overcoming differences'. Natsu is impulsive while Erza is more disciplined, and fanfictions might explore how they bridge these gaps in a romantic relationship.
Well, a big theme is the growth of their relationship. It doesn't have to be romantic in a sexual way. It can be about how they become closer as comrades. Also, the theme of protecting each other is quite common. Natsu is always ready to protect Lucy, and vice versa, and fanfictions like to play around with this dynamic, showing different scenarios where they safeguard one another.
One common theme is friendship. Both series have strong elements of friendship, and in fanfictions, it often gets explored further. For example, characters from different sides might become friends despite their initial differences.