Sasuke could also be an interesting choice. He is always seeking power, and a rat contract could offer him new ways to gain information about his enemies. He could use the rats' stealth abilities to his advantage in his quest for revenge.
Maybe the rat contract allows the user to communicate with rats in a special way. The rats could help in battles by distracting opponents or even stealing small but important items from the enemy. They could swarm over an opponent, causing confusion.
Naruto himself could be an interesting main character in a snake contract fanfiction. Imagine Naruto having to take on the snake contract for some reason, like to save Sasuke who has fallen deeper into Orochimaru's influence. It would be a huge change for him as he's always been associated with the toad contract. But with his determination, he could learn to master the snake powers in his own unique way.
It could be a new, young ninja who stumbles upon the cobra summoning contract by accident. This would be an interesting take as they start from scratch learning about the cobras and their powers.
Maybe it's an ancient evil that was sealed away long ago and the colossus contract is the key to its release. This evil force would be the main antagonist as it tries to break free and cause chaos in the ninja world. It could be something like a powerful demon that was once defeated by the first Hokage but is now on the verge of coming back because of the colossus contract. Naruto would have to stop it with the power of the colossus.
Naruto himself could be the main character. Given his adventurous and selfless nature, he might be the one to make the contract to save his friends or the whole Hidden Leaf Village. He would then have to deal with the consequences and powers that come with the contract.
A character like Rock Lee could be an interesting choice for the main character in a 'Naruto Girls Summoning Contract Fanfic'. He is known for his hard work and determination. With this summoning contract, he could use the skills of the summoned girls to complement his own taijutsu - based fighting style. Also, his positive and energetic personality would make for an interesting dynamic with the female characters he summons.
It could be someone who is jealous of Naruto getting the monkey contract. Maybe a rogue ninja who wants the power for himself.
The main antagonist could be an ancient snake spirit that was bound to the contract. This spirit might not like the idea of Naruto, a human, having control over the powers of the contract. So it tries to possess Naruto or drive him insane by constantly whispering dark thoughts and trying to make him use the snake powers for evil. It would be a battle of wills between Naruto and this malevolent spirit.
Naruto is a main character, known for his orange jumpsuit and his powerful Rasengan. Hana, who is often associated with animals, is also a main character. She adds an element of animal - related knowledge and skills. And M, the rat. M might be a unique rat that has a connection to some ancient power or prophecy. All three of them are central to the plot of the Naruto and Hana Rat M Fanfiction, with their own personalities and abilities driving the story forward.
It could be an ordinary woman living in a small town. She might be a kind - hearted person, like a teacher or a nurse. Her transformation into a rat would be a huge shock not only to her but also to the people around her.