One possible reason could be a threat to Konoha. If Iwa planned an attack on Konoha, Naruto, as a defender of his village, might take the fight to them and end up destroying parts of Iwa in the process.
It could also be due to a personal vendetta of someone in Iwa against Naruto or his friends. If they made a move on Naruto's loved ones, he would retaliate and his power could be so great that it results in the destruction of 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.
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.
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.
Maybe he feels ostracized in Konoha. If for some reason the villagers still don't fully accept him despite his heroics, he could be lured by the idea of a fresh start in Iwa.
Well, he might face a lot of challenges. The people in Iwa could be initially hostile as he's from Konoha. He'd have to prove himself. Maybe he'd learn new jutsu unique to Iwa's style.
Well, Naruto might face a lot of challenges in Iwa. He could encounter new types of jutsu that are unique to Iwa ninjas. Maybe he'll make some new friends or foes there.
Well, it could be that Naruto, for some reason, is abandoned in Iwa. Maybe he discovers some dark secrets about Iwa that make the villagers turn against him. He might then have to use his ninja skills and wits to survive and find a way out of the situation.
It could be that Kiba has been involved in something that Naruto considers wrong. For example, if Kiba endangered the safety of the village or their friends, Naruto, being the protector - type, would take action against him. Or perhaps there was a misunderstanding between them. Kiba might have been framed for something, and Naruto, believing the false information, tries to 'destroy' Kiba in the heat of the moment.
Maybe in the 'naruto neglected iwa fanfiction', there's a story where Iwa has a secret technique that could have been useful in the great ninja wars, but because they are neglected, they are not invited to share this knowledge. As a result, a young Iwa ninja goes on a journey to find a way to make the other villages recognize their worth and the importance of their techniques.
Well, one way Naruto could defeat Iwa in fanfiction is through strategic alliances. He could team up with other ninja villages or powerful individuals who have a grudge against Iwa. Together, they could plan elaborate attacks and outwit Iwa's military strategies. Also, Naruto's ability to inspire others might lead his allies to fight more bravely, giving them an edge in the battle.