Their relationship could develop through shared trauma. During the Pain Invasion, they face a great threat together. This might make them rely on each other more and understand each other better. For instance, Naruto might see Ino's courage in the face of danger, and Ino could see Naruto's selfless nature up close.
They could have friendly sparring sessions. Naruto, with his new - found strength, and Ino, with her mind - control jutsu, could engage in battles that help them both improve. These sparring sessions could also be a way for them to bond and understand each other better.
In fanfiction, they might become closer friends. Naruto's heroic deeds during the Pain Invasion could make Ino see him in a new light. She may start to respect him more deeply and offer him more emotional support. They could also train together sometimes, with Ino learning some of Naruto's unique ninja skills.
One plot idea could be that Naruto and Ino team up in a unique way during the Pain Invasion. Ino could use her mind - transfer jutsu to assist Naruto in predicting Pain's attacks. For example, she might be able to get inside the minds of Pain's summoned creatures and relay information to Naruto, giving him an edge in battle.
Naruto is, of course, very memorable. His determination to protect the village against Pain is always a central and inspiring part of any such fanfiction.
Naruto could also be more focused. The experience of the Pain invasion made him realize the importance of being fully present and concentrated during battles and training. He might have less of his usual goofiness and be more serious about his growth as a shinobi.
A great plot twist might be that some of the villagers who were thought to be innocent are actually in cahoots with Pain. They could have been secretly helping him all along, providing information or weakening Konoha's defenses from the inside. This would shatter Naruto's perception of the village and add a lot of drama and conflict to the story as he has to deal with both external and internal enemies.
After Pain's attack, Naruto and Ino could find themselves in situations where they rely on each other more than ever. Maybe they are assigned to the same team for a rebuilding project. At first, they might have some small disagreements, but as they work through them, they learn more about each other. Ino could use her skills to boost Naruto's confidence when he doubts himself. Naruto, in turn, could show Ino a different perspective on life and shinobi duties. This continuous interaction and mutual growth could lead to a very special relationship between them, whether it's a deep friendship or something more romantic.
In this 'naruto after pain invasion fanfiction', Naruto is central, of course. But characters like Jiraiya could be important even though he's dead. His teachings and memories could still influence Naruto's decisions. Iruka, Naruto's first teacher, might have a role in helping Naruto deal with his emotional state. Gaara, as Naruto's friend and the Kazekage, could come to Konoha to offer assistance and strengthen the bond between their two villages. And then there are the other members of Naruto's team, like Kiba and Shino, who could be involved in various tasks related to the post - Pain invasion situation in Konoha.