Gaara is a very likely recipient. He was a Jinchuriki like Naruto and they have a strong bond. Naruto might send him letters to check up on him and continue their friendship.
One possible scenario could be that the 5 - Tailed Jinchuriki has some sort of special power that draws both Itachi and Karin. Maybe Itachi is initially interested in the Jinchuriki for strategic reasons related to his Akatsuki duties, while Karin is intrigued by the unique chakra of the Jinchuriki. As the story unfolds, they all find themselves in a complex web of relationships and power struggles. Itachi might be conflicted between his loyalty to the Akatsuki and his growing feelings towards the Jinchuriki and Karin. Karin, on the other hand, could be using her own abilities to try and get closer to both of them for her own survival and perhaps a bit of romance.
The 'naruto jinchuriki of the world eater fanfiction' might center around the Jinchuriki's journey. Maybe they have to face challenges that come with being associated with a 'world eater'. This could involve battles against other powerful forces in the Naruto universe who either fear or want to use the power of the 'world eater' within the Jinchuriki. It could also delve into the Jinchuriki's relationships with other characters, like how Naruto's friends might react to this new and dangerous situation.
Often, other Naruto characters like Naruto Uzumaki or Kakashi Hatake might be included in the two - tails jinchuriki fanfiction. Their interactions with the two - tails jinchuriki can create new and exciting plotlines, whether it's them helping the jinchuriki control their power or going on adventures together.
Well, not having read the 'naruto jinchuriki of the world eater fanfiction', it's all speculation. The Jinchuriki is bound to be a main character. If it's set in the Naruto universe, Kakashi could also be in it. He might be in a position of trying to protect the Jinchuriki from those who would misuse the 'world eater' power. There could be a group of rogue ninjas who are after the Jinchuriki's power, and they could be major antagonists. And there may be a mysterious character who has a connection to the origin of the 'world eater' and has their own agenda regarding the Jinchuriki.
Well, it could be about Naruto somehow obtaining the power of Son Goku. Maybe there's a strange jutsu that fuses their powers. Naruto could use Son Goku's strength in battles, like having his speed and ki blasts along with his own ninja skills.
Yugito Nii is a very popular character as she is the actual 2 - Tails Jinchuriki in the Naruto series. In fanfiction, she often takes center stage, whether it's in stories about her training or her battles against powerful foes.
One interesting story could be about the two - tails jinchuriki's journey to control the beast's power. Maybe they face a powerful enemy and have to learn to use the two - tails' abilities in a new way to defeat them.
Perhaps the 5 - Tailed Jinchuriki has escaped from a hidden village and is on the run. Itachi is tasked with finding and capturing them. Karin, who is in the area for some other reason, stumbles upon the Jinchuriki first. She decides to protect the Jinchuriki out of her own sense of justice. When Itachi arrives, instead of immediately attacking, he starts to question his mission. The three of them then form an uneasy alliance to figure out a way to deal with the Akatsuki's plans for the Jinchuriki. Along the way, they face many challenges and their relationships grow. Karin starts to understand Itachi's complex nature and Itachi begins to respect Karin's tenacity, all while trying to keep the Jinchuriki safe.
It could be Orochimaru. He's always after power and seeing Naruto with Son Goku's power would make him a prime target. He might try to extract the power from Naruto for his own evil experiments.