Well, one interesting plot twist is when the Nine - Tails in Naruto is somehow linked to a powerful devil in Highschool DxD. Imagine the chaos and new power - sharing or battling that could occur. It completely flips the script on how we see both the Naruto and Highschool DxD elements. Also, there could be a plot twist where the power - leveling system in Highschool DxD is reversed for Naruto characters, making them struggle in a new way and forcing them to adapt in unexpected ways.
A great plot twist in some fanfictions is when the typical Highschool DxD setting of the school is transformed into a ninja academy - like place due to Naruto's influence. This changes the whole dynamic of the story as the DxD characters have to learn ninja ways and Naruto characters have to deal with the devils and angels in a different context. It also leads to some really interesting character development as they all adapt to this new situation.
One possible plot twist is that the hogyoku from Bleach somehow ends up in the hands of a character in Highschool DxD. This could lead to a character attempting to use it for their own gain, perhaps to gain ultimate power over both the devils and the angels. Another interesting plot twist is that a character from Highschool DxD gets reincarnated as a Soul Reaper in Bleach. They would have to learn the ways of the Soul Reapers while still retaining some of their previous abilities and memories. This could create some really interesting character development. A third plot twist could be that the balance between the three factions in Highschool DxD (devils, angels, and fallen angels) is disrupted by the arrival of Hollows from Bleach. The factions would have to figure out how to deal with this new threat, and it could lead to some unexpected alliances.
One interesting plot point could be the power exchanges. For instance, a character from Naruto might gain a devil fruit - like ability from Highschool DxD. Another could be the cultural clashes. The shinobi values of Naruto might be at odds with the more hedonistic values of Highschool DxD.
Another plot twist might be that the power of the Super Saiyan in the Highschool DxD universe has some unforeseen side effects. Maybe it attracts the attention of a new and extremely dangerous enemy that no one in the DxD world has ever encountered before. This would add a whole new layer of conflict and danger to the story.
A potential plot twist could be that the Kyuubi has a hidden connection to an ancient power in the Highschool DxD universe. Maybe it was sealed away long ago by the devils or angels and Naruto's presence awakens it. Another twist could be that Naruto is initially thought to be a threat by the main characters of Highschool DxD, but it turns out he is the key to solving a major problem they are facing, like a powerful enemy that is immune to their usual forms of attack but is vulnerable to Naruto's jutsu.
The most interesting aspect is the combination of different power systems. Naruto has his chakra - based ninjutsu, while Highschool DxD has the power of Devils, Angels, and Fallen Angels. When these two power systems interact in fanfiction, it leads to really cool battles and strategic situations. For example, Naruto's Shadow Clone Jutsu could be used in a very different way against the DxD characters compared to how it's used in the Naruto world.
A possible plot twist is that the characters from High School DxD initially underestimate Naruto due to his neglected state. But then they find out that he has a deep understanding of loyalty and friendship from his time in Konoha. This makes him form unexpected alliances in the DxD world, like teaming up with Issei to take on a common enemy that neither could face alone.
One interesting theme is the power - up concept. In Naruto, characters often gain new powers and abilities. When combined with the high - octane action and demonic powers of DMC and the unique abilities in Highschool DxD, it creates really exciting scenarios. For example, seeing a character from Naruto use Devil May Cry's combat styles while having the powers from Highschool DxD.
The most interesting plot could be one where Ranma and Issei swap bodies. Issei would have to deal with Ranma's martial arts world and his curse, while Ranma would have to handle Issei's life as a perverted devil. This would lead to a lot of comical misunderstandings and also some deep character development as they both learn from each other's experiences and try to get back to their original bodies.
Maybe in this fanfiction, the Nine - Tails in Naruto is actually a super - powerful Sacred Gear that was sealed within Naruto. This Sacred Gear has its own consciousness like in DxD. The story could revolve around Naruto trying to master this new form of power that is a blend of his chakra and the Sacred Gear's abilities. He would have to deal with the various factions in the DxD world who are interested in his power.
Well, a plot could revolve around a mystery that involves both worlds. Maybe there is an ancient artifact that has the power to disrupt the balance between the Naruto universe and the Highschool DxD universe. Characters from both sides have to work together to find and protect this artifact. This would lead to a lot of character development as they learn about each other's cultures and values while on this quest.