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 twist could be Naruto having to adapt to the unique magic system in Highschool DxD. Since Naruto has his chakra-based powers, seeing him struggle or find new ways to use his skills in a world of magic would be cool. For example, he might combine his shadow clone jutsu with some DxD magic to create a new and powerful technique.
One interesting plot could be the characters from Highschool DxD somehow ending up in the Bleach universe. Maybe Issei and his friends are mistaken for Hollows due to their demonic energy and are hunted by Soul Reapers. They then have to prove their innocence while also learning about the complex spiritual world of Bleach.
In some fanfics, a great plot twist is when Ichigo's Hollow powers are misinterpreted by the DxD characters as some sort of devilish power. This leads to a lot of misunderstandings and conflict at first. Another interesting plot twist can be when Ichigo somehow ends up being related to an important figure in the DxD world through some convoluted backstory.
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.
Another plot could involve the Quincy from Bleach. Uryu could be mistaken for a Devil by some of the Highschool DxD characters because of his unique powers. This misunderstanding could lead to a conflict at first, but then they realize they are all on the same side against a greater threat, like a powerful fallen angel who is trying to start a war between the Hollows, Devils and Quincies.
The combination of different power systems. In 'bleach highschool dxd fanfiction', you get to see the Soul Reaper powers from Bleach interact with the Devil and Angel powers in High School DxD. It's really cool to see how they balance or clash with each other.
One interesting plot twist could be a character from Bleach accidentally getting sent to Yokai Academy. Maybe Ichigo ends up there and is mistaken for a new yokai student. His reiatsu could be misinterpreted as some kind of strange yokai power, leading to all sorts of chaos as he tries to figure out how to get back home while also dealing with the unique challenges of the academy.
Perhaps a plot twist where the Geass power is revealed to be related to the Quincy powers in Bleach. This could mean that there is a deeper connection between the two universes. If Lelouch was part Quincy all along, it would change his character arc. He could then have access to Quincy abilities like Blut Vene and Blut Arterie, which would give him an edge in combat against both Britannian forces and any potential threats from the Bleach universe.
Ichigo and Rias would be a very interesting pairing. Ichigo is a powerful and determined fighter, and Rias is a strong - willed Devil. Their personalities could clash at first, but then they might find common ground in their leadership roles and their desire to protect their respective groups.
One of the most interesting plot twists could be the sharing of powers. For example, Naruto's ability to use chakra might combine with the unique demonic powers in Highschool DxD in unexpected ways. Maybe a character from Naruto's world gains a devil fruit - like ability from the Highschool DxD universe. Another could be a character from Highschool DxD learning how to use ninjutsu, which would completely change the power dynamics in their own world.