Maybe the zombies are not just mindless creatures. In this fanfiction, they could retain some of their former selves and memories. So, a zombie who was once a powerful magician could still use some of their magic, creating a whole new level of danger for the characters in Highschool DxD.
The zombie outbreak could be a result of a failed experiment by an evil organization within the Highschool DxD universe. This organization could be trying to create a new type of super - soldier but instead, they create zombies. The main characters then have to find a way to stop the spread of the virus and take down this organization at the same time.
One possible plot twist could be that some of the devils in Highschool DxD turn out to be immune to the zombie virus due to their demonic nature. This could lead to a power struggle within the group as they decide whether to help the humans or use the situation to their advantage.
I think Rias would survive. She's a powerful devil with a strong will. Her leadership skills and her ability to use her powers would help her fend off the zombies. Also, she has a group of loyal followers who would help protect her.
One interesting plot twist could be that the zombie outbreak is actually caused by a rogue ninja's experiment gone wrong. Maybe an old enemy of the Hidden Leaf Village who was trying to create a new type of weapon but instead created the zombie virus. Another twist could be that some of the zombies retain their ninja abilities. So, you'd have zombie - ninjas who could still use jutsu, making them much more dangerous. Or perhaps there's a cure hidden within an ancient scroll that only a few characters can decipher, adding a race - against - time element to the story.
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.
Issei could be the main protagonist. Given his determination and his ability to grow stronger, he could be in the forefront of fighting against the zombies. He might use his Boosted Gear to not only battle the undead but also to find a cure if there is some sort of infection involved.
A possible plot could be Venelana going on a solo mission to uncover a mystery that has been plaguing the underworld. This mission could lead her to discover new allies and enemies, and along the way, she could learn more about her own strength and capabilities.
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.
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.
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.
The plot could involve Ben's aliens having an unexpected effect on the magic in the Highschool DxD world. His alien DNA might interact with the devil or angel powers in strange ways. For instance, if he transformed into Heatblast near a magic spell, it could cause the spell to go haywire. And his presence could also disrupt the power balance between the different factions in the DxD world, leading to new alliances and rivalries.