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.
Perhaps they are hiding as part of a prank. Kiba from Naruto is known for his mischievous side, and Issei also has his moments. They could be hiding under Risa to scare her or to play a funny trick on her when she least expects it.
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.
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.
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.
One possible scenario could be that Naruto, with his shadow clone jutsu, creates a diversion that leads to an accidental explosion involving Kiba and his ninken. As for Issei, perhaps in a crossover setting, Naruto's Rasengan clashes with one of Issei's attacks and causes an explosion. However, this would require a lot of creativity to blend the two very different universes in a believable way.
Naruto could use his explosive - natured jutsus like the Rasengan. If he was sparring with Kiba, a misdirected Rasengan could hit something explosive nearby and cause an explosion involving Kiba. For Issei, if they were in a battle together, Naruto's chakra - infused attacks might react with Issei's energy - based powers in an unexpected way and create an explosion.
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 it could be about a competition between Naruto and Kiba during a particularly intense training session. They both get really 'in the zone' or 'in heat' in terms of their competitive spirit. Naruto might be using his new jutsu while Kiba is pushing Akamaru to their limits. This could lead to some friendly rivalry and growth for both of them.
The plot could involve Naruto having to adapt to Issei's world of devils, angels, and fallen angels. He'd likely use his ninja skills to stand out. Maybe he'd try to form a different kind of peerage, using his knowledge of teamwork from Konoha.
Well, one possible plot could be that Naruto discovers some ancient and powerful jutsu that allows him to reach across different dimensions or realms to revive Issei. Maybe Naruto is on a mission to save the world from a new threat and he realizes that Issei has some unique power or knowledge that can help. So he uses this jutsu, brings Issei back, and then they team up to face the common enemy.
It could be that they are in hiding to avoid a powerful enemy. This enemy might be after something they possess, like a powerful jutsu or a secret scroll. Naruto and Temari are forced to go into hiding, constantly on the move, and during this time, they develop a deeper understanding of each other and their own powers while trying to outwit their pursuer.