One interesting plot could be that there is a hidden scorpion summoning clan within the Naruto world. They have been in hiding for years, but now their secrets are starting to surface. The story could follow a member of this clan as they navigate the outside world and deal with those who would use their scorpion summoning abilities for evil.
One interesting storyline could be a young ninja who discovers a rare and powerful insect summon. For example, they might find a group of glowing beetles that can create illusions. The ninja then has to learn how to control and use these beetles in battles against other powerful foes in the Naruto universe.
One interesting concept could be unique jutsu related to the Scorpion bloodline. For example, the ability to control scorpion - like constructs for combat, like creating giant scorpion claws for offense or a scorpion - shell - like shield for defense.
Definitely Pikachu. Since Naruto is often in need of a quick and agile ally, Pikachu's speed and electric attacks would be very useful. Plus, Pikachu's cute appearance but powerful abilities would create an interesting contrast with Naruto's ninja style.
Well, a possible plot is that the lion summons are actually from a different dimension. Naruto accidentally stumbles upon the connection to this dimension and the lion summon contract. As he tries to understand and use this new power, he has to deal with the cultural differences of the lion society in that dimension. They could have their own rules and hierarchies. Another plot could be that the lion summons are the last hope to stop an ancient evil that is rising again. Naruto has to convince the lions to help him and the entire ninja world, which is no easy feat as the lions are initially reluctant.
One interesting plot could be a rabbit summon that has a special power to heal. In the fanfiction, Naruto could discover this rabbit during a mission in a hidden forest. The rabbit's healing power would be crucial in saving his comrades from a deadly poison.
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.
One interesting concept could be the unique and often dark nature of the summoned creatures. In regular Naruto, summons are somewhat standard, but in dark summon fanfictions, they might be more menacing, like demons with special powers that can corrupt the summoner or the environment around them.
One interesting plot could be that the serpent summon is actually a key to an ancient ninja treasure. Naruto has to go on a quest with his serpent to find the location of the treasure while fending off other ninjas who also want it.
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.
Maybe in the fanfiction, Naruto is in a situation where he accidentally summons a cobra during a crucial battle. The cobra turns out to be a really powerful and intelligent being. It helps Naruto not only win the battle but also starts to teach him some forgotten ninja arts. For example, the cobra could show Naruto how to use the environment to his advantage in a way that no one has thought of before.