The Third Tsuchikage himself might be a key character. His wisdom and power are well - known in the Naruto universe. In fanfiction, he could play a role in Naruto's interactions with Iwa, perhaps as a mentor - like figure to Naruto during his time in Iwa or as an obstacle that Naruto has to overcome to achieve his goals in the village.
Naruto is obviously a very popular character in these fanfictions as he is the main character of the original series. In Iwa, characters like Deidara (who was originally from Iwa) often make appearances. His explosive jutsu and unique personality add a lot to the stories.
There might be characters from Iwa who oppose Naruto. For example, the Tsuchikage of Iwa would surely be involved as the leader of the village Naruto is attacking.
Naruto could meet some young and upcoming Iwa ninja trainees. They might be curious about Naruto and his abilities from Konoha. These trainees could either look up to Naruto or be jealous of his fame, leading to various interactions. Also, there could be some old - timer Iwa ninjas who remember past events involving Konoha and have their own views on Naruto's presence in Iwa.
One possible reason could be a threat from Iwa to Konoha. If Iwa was planning an unprovoked attack on Konoha, Naruto might take it upon himself to destroy Iwa to protect his village.
He could meet some of Iwa's powerful ninjas. Maybe Onoki, who could either see him as a threat or an opportunity. There might also be some young Iwa ninjas who are curious about Naruto and his abilities.
In some fanfictions, Naruto defeats Iwa when he is on a mission to protect something important. Let's say Iwa is after a powerful artifact, and Naruto is tasked with safeguarding it. He uses his skills, along with the help of his friends, to fend off Iwa's attacks. During the battle, Naruto discovers a unique way to counter Iwa's jutsu by combining his own jutsu in an unexpected way. This gives him the upper hand and he manages to defeat Iwa. It's a classic underdog - turned - hero scenario that many fans like.
Maybe the main antagonist is the entire Iwa council. They might be the ones who order Naruto's abandonment because they fear his potential influence on Iwa. They would do everything in their power to keep him from uncovering their plans or causing unrest among the villagers.
A young and talented Iwa ninja who feels left out could be the main character. He or she might be frustrated with how their village is always in the shadow compared to Konoha and wants to change that perception.
Sasuke could be involved. He and Naruto often have intertwined stories. Sasuke might either be an ally in Naruto's destruction of Iwa, or he could be there for his own reasons, like seeking power or revenge.
One great Naruto Iwa Admiral fanfiction is 'The Stone Admiral's Ascent'. It has an interesting plot where the Iwa Admiral character shows unique jutsu and strategic thinking. It really delves into the character's backstory and how they interact with the Naruto universe's existing elements.
The main antagonists could be some of the conservative Iwa ninjas who oppose Naruto joining their village. They might see him as a threat to their traditional ways and values. Also, there could be some rogue ninjas from other villages who target Naruto because they see him as an easy target now that he's in a new and unfamiliar environment.