Another likely antagonist could be a rogue group within Naruto's village. They might have turned against the village's values and see Riasa as a means to an end, like using her as a bargaining chip with other villages or for some personal gain.
Well, perhaps the main plot could be centered around protecting Riasa from some sort of danger. Maybe there are enemies after her, and Naruto, Kiba and Issei have to use their unique skills to keep her hidden. Naruto could use his shadow clones to create diversions, Kiba with his dog companions could sniff out any approaching threats, and Issei might have some special abilities to help with the concealment.
Well, one possibility could be that they are hiding from some sort of danger. Maybe there's a powerful enemy in the Naruto world and in the Issei's world (assuming it's from High School DxD) that they need to avoid. And Risa's place is the only safe haven for the moment. They might be waiting for an opportunity to strike back or for help to arrive.
Another way to depict the fear is through their internal thoughts. The author could write in the first - person perspective for each character at times. Naruto might think about how the explosions remind him of the destruction he saw during the Pain invasion and how he doesn't want to see that kind of destruction again. Kiba could be thinking about how his ninja skills might not be enough to protect himself and his dog from the explosions. Issei might be having flashbacks to a past experience where he was nearly killed by an explosion and how that fear has now resurfaced.
Perhaps Naruto has a newfound confidence or a change in his personality that makes Kiba and Issei twitchy. In the fanfic, if Naruto starts acting more recklessly or assertively than usual, it might make them nervous. It could also be that Naruto has some sort of connection to an enemy or a dark force that Kiba and Issei sense, and that's what is making them twitchy.
One possible plotline could be that they are in a village or a world where there is a sudden increase in the use of explosive jutsus or weapons. Naruto, Kiba and Issei have to face their fear as they are constantly under the threat of these explosions. Maybe they start off by trying to avoid areas where explosions are likely to occur. For example, they might stay away from battlefields or places where ninja experiments with explosive substances are taking place. As the story progresses, they could be forced into situations where they have to confront their fear directly, like when their friends or loved ones are in danger in an explosion - ridden area. This would be a great opportunity for character development as they learn to overcome their fear for the sake of others.
At first, the relationship might become strained. Kiba and Issei might be a bit wary of Naruto because of whatever is making them twitchy. However, as the story progresses, they could start to communicate more. Naruto might realize that his actions are causing them discomfort and try to make amends. This could lead to a deeper understanding among them.
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.
One popular Naruto Shino x Kiba fanfic is 'The Bond Beyond the Surface'. It delves into their relationship as they go on a mission together and face unexpected challenges. The story focuses on how their different personalities complement each other during difficult times.
Kiba has his Inuzuka clan abilities. His connection with his dog, Akamaru, could be very useful. For example, Akamaru could be on the lookout while they are hiding, sensing any approaching danger long before they do. Issei, on the other hand, if we consider his abilities from High School DxD, he has his boosted gear which gives him power. He could potentially use it to defend their hiding place if they are discovered.