Well, one possibility could be that Naruto somehow enters the world where Nnoitra exists. Maybe he's on a mission to understand a different kind of power system that Nnoitra has. And through their interactions, Naruto starts to learn new combat techniques and ways of thinking. He might also try to change Nnoitra's violent nature with his own ideals of friendship and perseverance.
Obviously, Naruto and Nnoitra are the main characters. Naruto, the energetic and determined ninja from the Hidden Leaf Village, and Nnoitra, the powerful and often aggressive character from another world.
Adaptation is also a key theme. Nnoitra has to adapt to new environments, whether it's a different time period, a new world with different physics, or a society with different social hierarchies. This can lead to some really interesting character development as he tries to find his place in these new situations.
One common theme is the 'opposites attract' concept. Since Nnoitra has a rather rough and aggressive personality, the contrast with the reader's more normal or gentle nature is often explored. Another theme is the idea of redemption. Nnoitra may be seen as a character who can be redeemed through his relationship with the reader.
The contrast between their personalities. Nnoitra is often rough and aggressive, while Shinji can be more laid - back and mysterious. This clash can create really engaging storylines in fanfiction.
Some Tumblr blogs also focus on fanfiction. Follow some blogs that are related to the anime or manga where Nnoitra and Shinji come from. They might reblog or recommend some great Nnoitra x Shinji fanfictions.
One interesting crossover idea could be a battle tournament where characters from both Bleach and Fairy Tail participate. For example, Ichigo from Bleach could face off against Natsu from Fairy Tail. Their different fighting styles, with Ichigo's zanpakuto and Natsu's dragon slayer magic, would make for an exciting showdown. Another idea could be a team - up scenario. Maybe some of the Soul Reapers and Fairy Tail mages have to work together to defeat a common, powerful enemy. This would allow for interactions between characters who have different powers and personalities, like Rukia and Lucy. They could learn from each other's magic systems and grow as characters during the adventure.
A good way to create compelling character interactions is to play on the differences in their powers. Take Lucy from Fairy Tail and Uryu from Bleach. Lucy uses her celestial spirits, while Uryu has his Quincy powers. If they were to meet, they could initially be curious about each other's powers. Lucy might be amazed at the precision and control Uryu has over his arrows, while Uryu could be intrigued by the concept of celestial spirits. This curiosity could lead to them trying to learn from each other, or even having a friendly competition to see whose power is more effective in a certain situation. Through these interactions, their characters can grow and a deeper relationship can be formed.
One idea could be a tournament where characters from Bleach and Fairy Tail compete. For example, Ichigo from Bleach could face off against Natsu from Fairy Tail in a battle of strength and sword skills versus fire - based magic. Another plot could involve a common enemy that forces the two groups to team up. They start off distrusting each other but gradually learn to work together. Maybe a powerful dark force is threatening both their worlds and they have to combine their powers to defeat it.
It's hard to say for sure. Ichigo in his final form in Bleach has extremely high power levels. He can cut through almost anything with his sword and has great speed and strength. However, in Fairy Tail, Acnologia is a very powerful dragon slayer. His dragon form gives him incredible power and he has a lot of combat experience. So it could be either of them depending on how the fight is set up.