One interesting storyline could be Naruto using the takeover magic to assume the powers of different powerful ninja. For example, he might take over the Sharingan abilities from an Uchiha. This would create a whole new dynamic in battles and his relationships with other characters.
It makes Naruto more versatile. Instead of relying solely on his own chakra and ninja techniques, he can now use the powers he takes over. This forces him to adapt quickly to new abilities, which speeds up his character growth.
Well, one interesting idea could be the unique powers that come with the demon takeover. For example, the main character might gain super strength or the ability to control certain elements. Another aspect could be the internal struggle the character faces when the demon influence starts to take over their mind.
Naruto himself could be a main character in such fanfiction. Since he already has the Nine - Tails inside him, it's easy to imagine a story where he has a different kind of demon takeover. Another could be Sasuke. His quest for power might lead him to dabble in some dark magic involving demon takeovers.
Power struggle is a common theme. Naruto often has to fight to control the demon power that has taken over him, which creates an internal battle within himself.
One popular 'naruto demon takeover fanfiction' is 'Demon's Embrace'. It has an interesting plot where Naruto gains a unique demon power that changes the course of the story. The characters are well - written and it shows a different side of the Naruto universe.
Well, it can cause a lot of strain. Friends might be scared or confused by the sudden changes in the character who has the beast soul takeover. For example, if a normally calm and friendly character becomes aggressive due to the takeover, their friends might not know how to deal with them.
Madara Uchiha is another obvious candidate. Even though he was defeated in the original story, in a fanfic he could be resurrected with even greater power. His Sharingan abilities and his knowledge of the ninja world's secrets would make him a formidable antagonist for taking over the Naruto world. He could manipulate different clans and villages against each other before making his move to take over.
Yes, there are quite a few. You can check out fanfiction websites like FanFiction.net or Archive of Our Own. Some stories might explore Naruto's newfound soul magic abilities in relation to his existing ninja skills, like how it could enhance his Rasengan or his shadow clone jutsu.