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.
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.
One idea could be exploring the relationship between the 2 - Tails Jinchuriki and the other Jinchuriki in the Naruto world. Maybe they form an alliance to protect themselves from those who would use their power for evil.
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.
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.
It could be the Akatsuki. Even though Itachi was once part of them, now that he has the power of a Jinchuriki, they might see him as a threat. They could want to capture him to either extract the tailed beast or to stop him from using its power against them.
It could be Orochimaru. He's always been after power and Naruto having the power of two Bijuus would be very tempting for him. He might try to extract the Bijuus from Naruto to use for his own evil purposes.
Sakura could be a potential member. She's strong - willed and has her own set of skills. Hinata is also a possibility as she is kind and has always had feelings for Naruto. Ino could be in the harem too with her mind - related abilities. And Tenten, with her weapon skills, might also be an interesting addition.
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.