Maybe the fox ninken was once a normal ninken that was experimented on by some rogue ninja. Naruto stumbles upon it and through his kind heart and determination, he helps the ninken control its new fox - like powers. They then train together and the ninken becomes an important part of Naruto's team, using its speed and stealth to gather intelligence.
His friends might be a bit jealous at first. For example, Sasuke could see the fox ninken as another sign of Naruto getting stronger. But eventually, they would come to accept it. Shikamaru might be intrigued by the strategic possibilities the ninken brings to their team. Choji could be excited to have a new 'team member' to share food with.
Maybe the Ninken is a special one with some sort of prophecy attached to it. Naruto gets the Ninken and at first doesn't understand its true significance. But as time goes on, he realizes that the Ninken is key to unlocking a new power within himself. The Ninken also has its own backstory that is slowly revealed as the story progresses, like it was once part of a great ninja's team and has been waiting for the right person to come along, which is Naruto.
The Ninken could also have interactions with some of the village elders. They might be a bit cautious about this new addition to Naruto's entourage. But as the Ninken proves its loyalty to the village, they start to accept it. Naruto's relationship with the Ninken could also affect his interactions with the villagers. Some might be more afraid of Naruto now that he has this powerful animal by his side, while others might see it as a sign of Naruto's growing power and status in the village. And of course, there could be interactions with the various enemies Naruto faces. The Ninken could be a target because of its connection to Naruto, which would lead to some intense rescue and protection scenarios.
Often, they emphasize the loyalty between Naruto and his ninken. For example, in many such fanfictions, the ninken will stand by Naruto no matter how difficult the situation is.
In a 'naruto gets pregnant fanfiction', perhaps there is a powerful enemy who casts a spell on Naruto. This spell not only endangers his life but also has the side - effect of making him pregnant. The story could then revolve around Naruto trying to find a way to reverse this while also dealing with the reactions of his friends and the village.
The plot could revolve around Naruto being entered into a ninja beauty contest by Sakura as a joke. But Naruto, being Naruto, decides to take it seriously. So, he gets a total makeover. He starts training not just in combat but also in poise and grace. His makeover includes learning how to groom his hair better, wearing more refined ninja clothing, and even learning some basic etiquette. Along the way, he discovers a new side of himself and also manages to change some people's perception of him in the ninja world.
Well, it could be that Naruto discovers a hidden Pokemon in the Hidden Leaf Village. This Pokemon might have some special powers related to chakra. He then decides to train this Pokemon and enters Pokemon battles using his ninja strategies. He could also meet other Pokemon trainers who are interested in learning his ninja ways in exchange for Pokemon training tips.
One possible plot could be that Naruto discovers he has a hidden bloodline that gives him the ability to control time on a small scale. He has to learn to master this new power while keeping it a secret from his enemies. With this new bloodline, he could rewind a few seconds during battles to avoid attacks or foresee his opponents' moves.
In a fanfiction with such a title, it might be about Naruto coming across a character who is wrongly enslaved in a hidden part of the ninja world. Naruto, with his kind heart, decides to fight against the system that allows this slavery and in the process, the 'slave' character might end up being a great asset to Naruto in his future adventures, but not in an actual master - slave relationship but more of an equal partnership born out of mutual respect.
One possible plot could be that Naruto discovers a magical sword. This sword might have special powers like being able to cut through any jutsu. He then has to learn how to master it, which could involve going on a journey to find an old hermit who once used the sword. Along the way, he faces challenges from enemies who also want the sword for their own evil purposes.