It could be made more intelligent. Instead of just being a summon for combat, it could be a strategic advisor to Naruto, guiding him in his decisions with its ancient knowledge.
To write a good Naruto slug summon fanfiction, focus on the relationships. The bond between the summoner and the slugs should be strong and believable. For instance, they could communicate telepathically, sharing their thoughts and feelings. And don't forget about the conflict. Maybe there's a group that wants to control all the slug summons, and your characters have to fight against them.
In a fanfic, Naruto's teleportation jutsu could be different in terms of its range. Instead of having a set range, it could be based on his current chakra level. So, if he's full of chakra, he can teleport across the entire ninja world, but if he's low on chakra, it's only a short - distance teleport. Also, the jutsu could have a side effect. For instance, every time he teleports, he has a brief moment of disorientation. This would add an interesting element to battles and his overall use of the jutsu. Moreover, the teleportation could be linked to specific landmarks. He can only teleport to places he has a strong mental image of, like his home in Konoha or the training grounds where he practiced with Kakashi.
Ancalagon could also be shown as more intelligent. Maybe in fanfic, he's not just a brute force but a master strategist, planning his moves carefully in the stories. He could outwit other characters through his cunning.
One interesting Naruto slug summon fanfiction could be about a new character who has an extraordinary bond with the slugs. This character might use the slugs' unique abilities in creative ways during battles, like using their healing powers to revive an entire village after a major attack.
In terms of relationships, fem Naruto could have a more complex relationship with Sasuke. Maybe she doesn't have the same kind of one - sided crush as Naruto did in the original. Instead, they could start as rivals but slowly develop a mutual respect and understanding that is based on their shared experiences and growth as ninjas.
Nagato, in fanfiction, can be depicted in various ways. Some may show him as a more conflicted character. He has his own beliefs which were shaped by his experiences, like the death of his friends and the destruction he witnessed. He may be more hesitant to change compared to Naruto, but also has the potential for redemption which makes his character interesting.
Fuuinjutsu in Naruto fanfic often has different levels of complexity compared to the original. Fanfic writers may introduce new types of seals that have unique properties not seen in the show. For instance, a seal that can transfer a person's soul into another body. Also, the power levels of existing fuuinjutsu might be exaggerated in fanfic to create more intense storylines.
One idea could be exploring the relationship between Naruto and the slug he summons in more depth. Maybe the slug has a special power that only Naruto can unlock, and they have to go on a journey to discover it.
Their relationship could be portrayed as a mentor - mentee relationship in a different way. Instead of Orochimaru trying to use Naruto for his own gain, he could genuinely see Naruto's potential and guide him in the ways of some of the more esoteric jutsu. For example, Orochimaru could teach Naruto about the secrets of the snake contract without any ulterior motives. Another way is to show them as reluctant allies. Maybe there's a common enemy that is so powerful that they have no choice but to work together, and over time, they start to respect each other's abilities.
In some fanfictions, the Naruto - Sakura relationship is portrayed as a slow - burn romance. Naruto's continuous efforts to protect Sakura gradually make her fall for him. In contrast, in the original series, Sakura was mainly focused on Sasuke. Fanfiction gives an opportunity to change that narrative.