It could be a resurrected Orochimaru. He has always had an interest in power and the Akatsuki. He might return, more powerful than ever, and take control of the Akatsuki in the Boruto era, using them to carry out his evil plans, like creating an army of super - ninjas using some dark jutsu.
Perhaps a new character entirely. A rogue ninja from a hidden village who has a grudge against the current ninja world order. He could form his own version of the Akatsuki, with members who share his views. This new villain would be cunning and powerful, with a goal to overthrow the existing shinobi system and rule the world himself.
There's also the possibility that an old Akatsuki member, like Kisame, could be brought back in a different form. Maybe he was resurrected by some dark magic but is now more of a wraith - like being. He could be the main villain, still having loyalty to the Akatsuki's old ways and seeking revenge on those who defeated the original Akatsuki.
The main villain could be a big, mean cat from the neighborhood. This cat is always trying to steal Sakura and Akatsuki's food and toys. It's territorial and doesn't like the two little kittens around. It chases them away from their favorite spots to sunbathe and play.
It could be a new character entirely. Maybe a rogue ninja from a long - forgotten clan who holds a grudge against Naruto for some past event. This villain could be extremely powerful in dark jutsu and sees Naruto's death as the first step in his plan to take over the ninja world.
Konan could be the main character. She might be the one trying to bring some Christmas spirit to the otherwise dark - natured Akatsuki. She could use her paper - making abilities to create beautiful Christmas decorations for their hideout and convince the others to participate in some small Christmas traditions like exchanging small gifts.
A fanfic writer could make it believable by giving Boruto a strong motivation. For example, if Akatsuki somehow manages to kidnap his friends or family and he has to join them to ensure their safety. Another way is to show that Boruto is being misled. Maybe Akatsuki presents themselves as a group with a noble cause at first, and Boruto, being young and naive, falls for it. This would make his joining seem more plausible.
Well, if Boruto joins Akatsuki in a fanfic, it would be a huge shock to the Naruto universe. Boruto is from a new generation and Akatsuki was the main antagonist group in Naruto's time. He might bring new skills and a different perspective to the Akatsuki. Maybe he could try to change the Akatsuki from within, or he might be forced to do their bidding against his will at first. It could lead to some really interesting storylines, like how the existing Akatsuki members would react to this new young recruit.
Another concept is the Akatsuki's relationship with the new generation in Boruto. They might try to recruit some of the young and powerful ninjas. This could lead to a conflict within the new generation, as some might be tempted by the Akatsuki's power and ideals, while others firmly oppose them.
It could be a powerful warlord who has been tracking Yona's group and follows them into the cave. He wants to capture Yona to use her as a bargaining chip against the Kouka Kingdom. His army is large and well - trained, and he is cruel and cunning, always devising new ways to try and outwit Yona and her friends.
The main characters could be the leaders of the Akatsuki group at the high school. They would be the ones making the big decisions and leading the group in various school - related activities. For example, they might be the captains of different school clubs that the Akatsuki group is involved in.
Sarada could also be a main character. She is strong - willed and intelligent. She could be investigating a case of a missing - nin who is suspected of being involved in some sort of conspiracy against the Uchiha clan. This would give her a chance to show her detective skills and her loyalty to her clan.
Maybe an ancient evil spirit that has been awakened due to some mistake. It targets Ageha because she has a pure and powerful chakra. Naruto, being the hero he is, tries to fight the spirit, but he quickly realizes that he needs Ageha's help as well as the help of all his friends to defeat this powerful foe.