Maybe it's a new character introduced. A girl or boy from a far - off land who has a frozen heart because of some dark magic or past experience. They come to the Naruto world and Naruto himself takes it upon himself to befriend them and melt their frozen heart.
It could be Sasuke. He has been through a lot of trauma, especially with the massacre of his clan. This could have led to him having a sort of frozen heart, being cold and distant. The fanfiction could explore his journey of slowly opening up again.
Another possibility is Kakashi. He has his own share of past traumas, like losing his teammates at a young age. This could have caused his heart to be a bit frozen in terms of fully expressing his emotions. The fanfiction could be about how he finally breaks free from that emotional block with the help of Naruto and the others in the village.
Another possibility is Gaara before he met Naruto. He had a really tough childhood and was initially very cold - hearted, seeing others as tools or threats. His heart was like a frozen desert, until Naruto's friendship started to thaw it. In this fanfiction, they could explore more of his pre - Naruto state of a frozen heart and how it affected his relationships with others in the shinobi world.
Well, it could be about a character in the Naruto universe who has a frozen heart, perhaps metaphorically. Maybe it's a ninja who has been through so much pain and loss that they've become cold and unfeeling. And the story could be about their journey to thaw that frozen heart, maybe through the friendship of Naruto and his gang.
Madara Uchiha might be the main villain. He has powerful time - related abilities in the original series. He could use his Mangekyo Sharingan or other powers to freeze Naruto in time. His goal could be to disrupt the balance of power in the ninja world. With Naruto out of the picture, he can more easily carry out his plans to reshape the world according to his own vision. He might see Naruto as a major obstacle to his grand plan, so he freezes him to eliminate the threat.
A new character could be introduced as the main antagonist. For example, a powerful ice sorcerer who is jealous of Jack Frost's natural ice powers. This sorcerer might have developed a special spell to freeze Jack's heart in order to steal his powers and take over his role as the bringer of winter.
It could be a new ice - using ninja who has a grudge against Naruto. Maybe this ninja wants to prove himself as the most powerful ice user and sees Naruto as a threat, so he freezes Naruto in ice.
Captain America might be the main character. He represents the heart and soul of the Avengers in many ways. In this fanfiction, he could be the one leading the team through an emotional crisis, using his unwavering moral compass to guide them and protect the heart of the team and the world they are trying to save.
It's hard to say for sure without reading it, but it could be Naruto himself. Since it's a Naruto - related fanfiction, he might be the central figure dealing with the frozen flame concept.
Since I don't know the exact 'frozen heart novel', it's hard to say. But it could be a young girl and a mysterious man with a frozen heart.
One interesting concept could be the fusion of Naruto's world with the idea of a frozen heart. Maybe it explores how a character with a frozen heart, perhaps a new villain or an existing character turned cold, interacts with Naruto's warm - hearted and determined nature. It might also play with the idea of thawing the frozen heart through Naruto's unwavering belief in friendship and redemption.
Another possibility is that there is no real 'antagonist' in the traditional sense. Instead, it could be the circumstances that act as the 'antagonist'. For example, the differences in their worlds, their duties, and the fact that they come from such different backgrounds could be what causes Erza to break Naruto's heart.