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.
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.
In fanfiction about Naruto and Demon Slayer with magic, one interesting aspect is the potential for new power - ups. For instance, a character could combine the Sharingan from Naruto with a Demon Slayer's special vision ability and create a super - powered form of sight. Also, the magic could be used to explain some of the more mysterious elements in both series. Maybe the curse marks in Naruto are actually a form of suppressed magic, and in the fanfiction, a character figures out how to fully utilize this magic, taking on a new and powerful form inspired by Demon Slayer's demon - slaying forms.
Magic in this type of fanfiction can be used to bridge the gap between the two universes. It can make the transition of characters from one world to another more believable. For example, if Naruto characters enter the Demon Slayer world, magic could be the reason they can adapt to the new rules and powers there.
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.
Demon magic in these fanfictions usually has a corrupting influence. It could work by tapping into the dark energies of a demonic realm. The caster may have to perform elaborate and often cruel rituals. For instance, they might need to use the blood of magical creatures or the souls of the damned. This magic is not like the normal magic in the Harry Potter world. It defies the laws of normal magic and often has unpredictable and dangerous consequences. It can also gradually corrupt the caster's mind and body, making them more and more like the demons they are drawing power from.
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.