The mystery surrounding the characters. Tobi is a very enigmatic figure, and Kakashi has his own secrets. When combined in fanfic, it creates a lot of intrigue.
One popular 'tobi x kakashi fanfic' could be 'The Unseen Bond'. It explores a unique relationship between the two characters, delving into their shared past and how their paths cross in unexpected ways. The story is filled with mystery and a slow - burn kind of connection.
Naruto would probably be one of the first to try and rescue Kakashi. He has a strong bond with Kakashi and would not stand by while his teacher is in danger.
One possible plot could be that Tobi kidnaps Kakashi to gain some valuable information. Maybe he wants to know about the Sharingan secrets Kakashi has. Tobi could hold Kakashi in a secret lair and try to interrogate him.
In this fanfiction, perhaps Kakashi and Tobi meet in a battle. Kakashi might initially be suspicious of Tobi's true identity. Tobi could be playing mind games with Kakashi, trying to throw him off his game.
The setting also plays a big role. Whether it's in the original Naruto universe or an alternate one. In some 'tobi x naruto fanfic', the authors create new locations or change the existing ones to fit their story. And of course, the character backstories are important. Tobi's mysterious past and Naruto's journey to become Hokage are often used to add depth to the plot.
A very common theme is the clash of personalities. Tobi is often depicted as this somewhat unpredictable and shadowy figure, while Sakura is strong - willed and determined. So, their interactions are full of tension. Also, the theme of trust - building is quite prevalent. In many fanfics, Sakura has to learn to trust Tobi despite his mysterious nature.
A third theme could be growth. They both start from certain points in their characters and develop over the course of the fanfic. Tobi might become more serious or Zetsu might open up more, depending on the plot and how the author wants to develop their relationship.
Kakashi could become more distrustful after meeting Tobi. Since Tobi is such a mysterious figure, Kakashi might start to suspect everyone around him more.
One great 'tobi x sakura fanfic' is 'The Unlikely Bond'. It explores a unique relationship between Tobi and Sakura, with a plot full of mystery and some really interesting character interactions.