The older brother in the fanfiction might be a character with a tragic past. For example, he could have been experimented on by Orochimaru and gained some strange powers as a result. And he returns to the village later to protect Naruto from the threats that are coming towards him.
In the fanfiction, the older brother could change Naruto's view of his family and his place in the world. Naruto would have a connection to his past that he didn't know before. This could lead him to explore more about the Uzumaki clan and discover hidden powers or knowledge. And it could also affect his relationships with Sasuke and Sakura. For instance, his brother could help him understand Sasuke's pain better, leading to a different dynamic between them.
Well, it could be about the fox brother having special powers. Maybe he can communicate with Naruto in a unique way, like through telepathy. They might go on adventures together, facing off against enemies that Naruto couldn't handle alone. The fox brother could also have a connection to Naruto's own inner fox spirit, and they could work together to control that power better.
Another possibility is that he could be the brother of Weiss. Weiss comes from a wealthy and proper family. Naruto's more free - spirited and unorthodox ways would clash with Weiss's upbringing at first, but they could grow to understand and support each other as the story progresses.
It could be a powerful ninja who wants to capture the fox brother for his own purposes. Maybe he believes that the fox brother's powers can be used to take over the ninja world. This antagonist might be from a rival village or a secret organization within Naruto's own village.
At first, Naruto's little brother might idolize Naruto. However, as he grows older, he might start to resent Naruto a bit because of the pressure of being related to the famous ninja. But through some difficult situations, like fighting together against a powerful enemy, they could come to understand and respect each other more deeply. Naruto could learn to be more responsible as an older brother, and the little brother could learn to see Naruto as a person with flaws and not just a hero. This would lead to a more complex and loving relationship.
One possible plot could be that the twin brother has a very different personality from Naruto. Maybe he is more reserved and studious while Naruto is brash and energetic. The twin could secretly be training in a different type of jutsu, perhaps more focused on genjutsu while Naruto is into rasengan and other offensive ninjutsu. Their relationship might start off rocky as they compete for recognition in the village, but eventually they come to realize they are stronger together and support each other in battles against powerful enemies.
Yes, there are likely fanfictions out there with such a concept. Fanfiction writers often come up with creative and unexpected crossovers like this. You can search on popular fanfiction websites such as Archive of Our Own or FanFiction.net to find stories based on this idea.
It could be an unknown brother that was kept secret all this time. Maybe he was hidden away by some enemy of the village and trained to be a powerful adversary for Naruto.
Perhaps Naruto's brother could be a long - lost relative from a far - flung part of the ninja world. He could have been separated from Naruto when they were young due to a war or some political intrigue in the ninja clans.