A cool concept would be an OC with multimagic that allows them to travel through different dimensions. In the Fairytail world, this could lead to all sorts of adventures. They might accidentally bring back a powerful and dangerous entity from another dimension, and the Fairytail guild has to team up with the OC to send it back while also dealing with the OC's insecurities about their strange magic.
One idea could be to create an OC (Original Character) who has a unique multimagic ability that is related to the elements but in a different way than the existing characters in Fairytail. For example, instead of controlling fire, water, etc. directly, the OC can manipulate the energy within those elements. They could join Fairytail and face challenges that test their new abilities and their loyalty to the guild.
To develop the OC's character in a multimagic Fairytail fanfic, focus on their goals and dreams. If their magic gives them special powers, what do they want to do with it? Do they want to use it to protect the weak or gain power? Their relationships are also crucial. For instance, if they fall in love with a Fairytail member, how does that affect their actions and decisions? You can also give them flaws, like being too impulsive when using their magic, which makes them learn and grow throughout the story.
You could have an OC who is a gamer in the real world but gets transported into the Fairytail universe. At first, they struggle to adapt but then realize that their knowledge of games can help them understand the magic system better. For instance, they might compare the different types of magic to character classes in RPGs and use this knowledge to develop their own unique magic skills.
Well, here's a plot idea. The male OC has a special connection to a magical artifact that is coveted by an evil force. He has to protect it while also trying to figure out why he has this connection. Also, he could be in a love triangle with two other characters in the guild, which adds a lot of drama to the story. Another idea is that he's the key to unlocking a hidden power within the Fairytail guild, but he doesn't know it at first and has to go through a series of trials to discover his true potential.
Another idea is to have the OC be from a different dimension where Jean Grey has a different role or even doesn't exist. The OC then gets transported to the X - Men Evolution universe. They have to adjust to this new world while also trying to find a way back home. Along the way, they cross paths with Jean Grey and form an unlikely friendship. This could lead to a lot of interesting interactions as the OC tries to understand Jean's powers and the X - Men's way of life.
One idea could be creating an OC who is a long - lost relative of a main character. For example, an OC could be Natsu's sister who was separated from him at a young age. This OC could have unique fire - based magic that is different from Natsu's. Another idea is an OC from a different dimension that gets trapped in the Fairytail world. They could bring new types of magic and technology from their own world, causing chaos and new adventures.
Another storyline could involve Kai's relationships. He could fall in love with an OC from a different tribe. This would create a lot of drama as their tribes might have some sort of feud or different values. They would have to fight against the odds to be together, and Kai might have to prove himself to her tribe to gain their acceptance.
One idea could be to create a male OC who is a skilled mechanic. He could start off as an ordinary worker in one of the Gundam 00 facilities. But then, through a series of events, he discovers a hidden ability within himself related to the Gundam technology. Maybe he can communicate with the Gundam on a deeper level than anyone else. This could lead to him being recruited into a special team that deals with high - risk missions. He would have to face challenges not only from the enemies but also from within the team as some might be jealous of his new - found abilities.
One idea could be an OC who is a long - lost relative of the Russo family. They come to town with a special power that the Russos don't have, like the ability to control time. This could create all sorts of interesting situations, such as going back in time to prevent a major disaster that was going to happen to the family in the future.
Perhaps an OC Lannister who is a talented healer. In a world full of wars and battles in Game of Thrones, a healer Lannister could be an interesting addition. They might have to deal with the moral dilemma of healing both their own family's enemies and allies. They could also have a secret love affair with a member of another house, which would add a layer of drama.
Another idea is that the OC is a mage who can transform into a cat - like Pantherlily. But they are not fully in control of this power. Pantherlily takes it upon himself to train the OC to master this ability. Along the way, they get involved in a guild - wide competition where the OC has to prove their worth not just as a cat - like mage but also as a true Fairy Tail member.