The unique feature is often the new powers Naruto gets. For example, in many 'possessed Naruto fanfic', he might gain dark or ancient powers that are not part of his normal abilities in the original series. This can lead to very different fight dynamics.
One popular 'possessed Naruto fanfic' is 'The Possessed Jinchuriki'. In this fanfic, Naruto is possessed by an ancient spirit which gives him new and unexpected powers. It has an interesting plot that explores how Naruto copes with this new entity within him while still trying to protect his village.
Another plotline might involve Hermione being possessed by a time - traveling consciousness. This allows her to experience different eras of Hogwarts' history first - hand. She could then bring back important information from the past to help in the present, like how to defeat a powerful dark wizard that is starting to rise again in their time.
One popular 'Naruto Possessed' fanfiction could be 'Naruto: Possessed by the Nine - Tails' Power'. It often explores how Naruto's already formidable power is enhanced and changed by a more intense connection to the Nine - Tails. Another might be 'Naruto: Possessed by a Mysterious Force' which delves into how he copes with a new, unknown power within him. And 'Naruto: Possessed by the Spirit of an Ancient Sage' where he gains the wisdom and abilities of an ancient being.
Another theme might be the discovery of new powers or abilities that come with the possession. It could add an interesting twist to the normal Ben 10 power set. Also, there could be a theme of redemption if the possessor is initially a villain. The story could be about how the possession changes the relationship between Ben and his friends and family as they try to figure out what's wrong and how to fix it.
Often, when possessed by a dark force, she'll be shown as more menacing. Her eyes might change color, and she'll have an air of danger around her. Authors will make her act cruelly towards others, especially those she used to be friends with. This is a way to show the contrast between the normal Hermione and the possessed one.
Well, in a 'naruto possessed by evil fanfiction', Naruto could be made to do all sorts of uncharacteristic things. His sense of justice could be warped, leading him to believe that the only way to achieve peace is through domination. He may also develop new and dark jutsu, and his relationships with characters like Sasuke and Sakura would be severely strained as he becomes a threat to them.
Obviously, Naruto is the main character as the story revolves around him being possessed by the Kyuubi. Sakura and Sasuke could also play important roles. Sakura might be worried about Naruto and try to find a way to help him break free from the possession. Sasuke could be either an ally or an antagonist depending on the plot. He might see Naruto's new - found power as a threat or a means to achieve his own goals.
One common plotline is the struggle for control. Naruto has to fight against the entity possessing him, like in many fanfictions where he battles his own body's takeover. Another is the reaction of his friends. They usually try to find a way to save Naruto, which often involves researching ancient texts or seeking out powerful ninja who might know a solution.