Perhaps in the fanfiction, the leviathan is a sentient being from a different dimension. Naruto forms a bond with it not just as a summon but as a friend. They communicate telepathically, and the leviathan helps Naruto understand some of the deeper secrets of chakra by showing him how it manipulates energy in its own dimension.
A cool concept could be that the leviathan is the last of its kind. When Naruto summons it, he has to protect it from other ninja clans who want to capture it for its power. The leviathan in turn teaches Naruto some new and powerful jutsu as a way of thanking him for his protection.
Maybe the Cerberus in the fanfiction has a special connection to the Nine - Tailed Fox. It could be that they are both from an ancient and powerful lineage. The Cerberus could be a guardian of some sort, and its summoning requires the main character to prove their worthiness, perhaps by passing a series of trials in a mysterious realm.
An idea could be that the crows have a hidden language that only a select few can understand. The protagonist of the fanfiction is on a quest to decipher this language and discovers that it holds the key to a long - lost jutsu. This would involve a lot of mystery and exploration as they interact with the crows.
One idea could be a crossover where Naruto characters end up in the Highschool DxD world and have to deal with the devils and angels there. For example, Naruto could use his shadow clone jutsu in a battle against a group of devils, creating chaos. Another plot idea might be that a Leviathan character from Highschool DxD travels to the Naruto world and tries to learn ninjutsu. It could lead to some comical situations as they struggle with chakra control.
Maybe the chameleon is from a long - lost clan of summon animals that were thought to be extinct. And it holds the key to a powerful jutsu that was lost in time. Naruto discovers this when he first summons the chameleon and has to go on a journey to unlock the jutsu's potential.
One idea could be a story where Naruto discovers a new type of frog summon that has a unique ability, like the power to control water in a way no other frog can. This could lead to him facing new challenges and enemies. Another plot could involve Naruto's frog summons going on a mission without him, and he has to find a way to communicate and guide them from afar. Maybe they encounter a powerful enemy in the frog realm that they need Naruto's help to defeat.
One idea could be that Naruto discovers a hidden dragon summoning scroll. He tries to master the summoning technique, but the dragons are very powerful and difficult to control at first. Another plot could involve a dragon that has a special connection to Naruto's family history. Maybe his mother or father had a pact with the dragons long ago.
One idea could be a human summon from a long - lost clan with unique powers. For example, a summon from a clan that can control time in small increments. This human summon could be an ally to Naruto in a difficult battle against a new, powerful enemy.
Well, there could be a 'naruto leviathan fanfiction' where the leviathan is actually a form of a powerful jutsu gone wrong. Naruto and his friends have to find a way to reverse the jutsu and stop the leviathan from wreaking havoc. It could involve a lot of teamwork and Naruto using his new - found skills to combat the beast. This type of story would be full of action and adventure as they race against time.
Perhaps the Knights of the Round are summoned by accident when Naruto is trying out a new experimental jutsu. They initially misunderstand their purpose in this new world and cause some chaos. But as they learn about Naruto's world and his cause, they decide to help him in his fight against the Akatsuki. They could offer strategic advice based on their own experiences in their home realm.
One idea could be that Naruto summons a special fox with unique powers that helps him defeat a powerful enemy in a new and unexpected way. For example, the fox might have the ability to create illusions so strong that they can mislead even the most powerful ninjas. Another plot could involve Naruto using the summoned foxes to explore a hidden part of the ninja world that was previously inaccessible. Maybe the foxes know the secrets of an ancient ruin and lead Naruto there, where he discovers new jutsus or artifacts.