Well, it could be that the small fairy is cursed and turns into a lemon - like being every time the sun sets. The man, who has a special ability to see through curses, tries to break it. He uses his knowledge and the help of some magical items in the fairy world. Their relationship grows as they work towards breaking the curse and restoring the fairy to her normal self.
One idea could be that Spider Man and Wasp team up to fight a new villain that is causing chaos in the city. Their partnership gradually turns into a romantic relationship as they discover more about each other during their battles. Another plot could involve them getting trapped in an alternate dimension where they have to rely on each other to find a way back home, and during this time, their feelings for one another deepen. A third plot might be that Wasp is in danger and Spider Man has to go to great lengths to save her, which makes her see him in a new light and start a relationship.
A possible plot idea is that there's a curse on the Fairy Tail guild related to lemons. Whenever a lemon is present, some members start acting out of character. This could lead to humorous situations as well as some serious challenges for the guild to overcome. Or perhaps there's a lemon - based competition in the magical world that Fairy Tail decides to enter. They have to use their magic in unique ways to succeed in tasks related to lemons, like making the biggest lemon - shaped structure using magic or finding the rarest lemon in a magical forest.
Perhaps a plot where a powerful villain threatens both the Naruto and Fairy Tail universes. The characters then combine their skills, like Naruto's ninjutsu and Fairy Tail's magic. In the harem part, it could be that the main character, say a new character created for this fanfiction, has to navigate the different personalities and affections of characters from both series while they all work towards saving their worlds.
The man can be developed by showing his initial reaction to the small fairy. For example, if he's shocked at first but then becomes curious. His character could grow as he learns more about the fairy's world and starts to care for her. As for the small fairy, her character can be developed through her interactions with the man. If she's initially shy but gradually becomes more open and trusting.
One idea could be Virgo using her earth - magic related abilities in unique ways within the fanfiction's plot. For example, she could create underground passages to help the Fairy Tail guild members escape from a dangerous situation. Another plot idea might involve Virgo's relationship with other characters in a more comical way, like her trying to serve them in the most unexpected situations.
Perhaps a fanfiction where a demon has a unique power that is needed by the Fairy Tail guild. The demon is initially reluctant but then realizes the importance of helping. They could form friendships within the guild and have adventures together, like going on a quest to retrieve a magical artifact.
A plot could be that the Great Fairy Lemon has lost her memory due to a magical accident. Link discovers her in this state and decides to help her regain her memories. As they journey together, they face enemies that are after the weakened Great Fairy. Link must fight them off while also trying to find the key to restoring her memory, which could be hidden in an ancient, forgotten temple.
A plot could be where the reader has a unique magic ability that Natsu has never seen before. Natsu is intrigued and they start to explore how this new magic can be used in combination with his fire magic to defeat a new, powerful enemy that threatens the Fairy Tail guild. It would be a story of discovery and teamwork.
One idea could be that they accidentally swap powers. Spider - Man suddenly has Deadpool's regenerative abilities and his crazy sense of humor, while Deadpool has Spider - Man's spidey - sense and web - slinging skills. They have to figure out how to deal with these changes while still fighting crime. It would be a humorous and action - packed plot.
Maybe an OC who has the ability to communicate with spirits. In the 'fairy tail x oc fanfiction', this OC could discover that there are ancient and powerful spirits that are about to be awakened due to some dark magic ritual. The OC tries to enlist the help of the Fairy Tail guild to stop this from happening. Along the way, they face many challenges and also have to deal with the guild's initial skepticism about their powers and the existence of these spirits.