Obviously, Naruto is one of the main characters since he is the one sealed in time. His friends like Sasuke, Sakura, and Kakashi are likely to be main characters too as they would be involved in trying to save him.
Maybe the village has changed a great deal while Naruto was sealed. When he finally breaks free, he has to face a whole new set of challenges and enemies that have emerged in his absence. There could be new political situations in the Hidden Leaf Village as well.
Naruto is surely one of the main characters since he is the one being sealed. His friends like Sasuke, Sakura, and Kakashi might also be main characters as they would likely be involved in the events related to Naruto's sealing.
In many such fanfictions, it could be a variety of deities. It might be an ancient and powerful god from Naruto's world's mythology that the story's author has chosen to insert. For example, it could be a god of elements like a super - powerful fire god, which could give Naruto even more control over his existing fire - based jutsu and new, more extreme abilities related to fire.
In some fanfictions, Kushina was sealed into Naruto through a complex jutsu. It might be a modified version of the sealing jutsu used to seal the Nine - Tails. The process could involve special incantations and chakra manipulations.
In this fanfiction, perhaps an interesting plot point is the discovery of the reason behind Naruto being sealed in time. It might be related to some ancient prophecy or a powerful enemy's plot. Also, the development of new characters who are affected by Naruto's absence in time could be really engaging.
Perhaps an old enemy has returned, stronger than ever. They might have been plotting during Naruto's sealing and now are ready to take on Naruto and the Hidden Leaf Village. This could be someone like Orochimaru who found a new way to gain power in Naruto's absence.
One interesting aspect could be how Naruto's character has evolved during the long - time sealing. Maybe his skills have developed in unique ways or his relationships with other characters have deepened in the context of this fanfiction.
In some Naruto fanfictions, Madara might be sealed through a powerful jutsu created by the main character. For example, the protagonist could combine different elemental chakra natures to form a unique sealing technique specifically designed to counter Madara's power.
Well, in some Naruto fanfictions, Bardock might be sealed by powerful jutsu. Maybe a dark ninja uses a forbidden sealing technique to lock him away, preventing him from interfering with their evil plans.