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.
Rias could also make a great main protagonist. As a powerful devil, she could lead her peerage in the fight against the zombie menace. Her strategic thinking and powerful magic could be key in devising plans to stop the spread of the zombies and protect the school and the town.
Issei could still be the main protagonist. His determination and ability to grow stronger could play a big role in any story involving Trihexa. He might be the one to face Trihexa head - on or try to befriend it.
It could be a new student who discovers they are born of fire. Maybe they were adopted and had no idea about their true nature until they start experiencing strange powers during their time at the Highschool DxD. This character would then be thrust into the world of devils, angels, and other supernatural beings.
Maybe a rogue ninja from Gaara's world who somehow follows him into the DxD highschool. This ninja could have a vendetta against Gaara and use the chaos of the DxD world to try and take him down. They could also team up with some of the devils who dislike Gaara's presence to create more trouble for him.
It could be a corrupted member of the mafia who has made a deal with a rogue group of fallen angels. This person betrays his own mafia family and works with the fallen angels to create chaos in the Highschool DxD universe. He is very sly and manipulative, always staying one step ahead of the heroes. For instance, he uses his knowledge of the mafia's operations and the power of the fallen angels to set traps for the protagonists.
Issei could be an interesting main character in a 'Highschool DxD Akatsuki' fanfiction. He already has his own journey in the Highschool DxD world, and if he were to interact with the Akatsuki, it would add a whole new dimension to his story. He could learn new techniques from them or even form an alliance against a common enemy.
The devils who oppose Gilgamesh's presence in the Highschool DxD world could be main antagonists. They might see him as a threat to their power and influence in the school.
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.
Maybe a rogue Devil who wants to use Ichigo's power for his own gain. He could try to manipulate Ichigo into fighting against the other factions, like the Angels and the Devils who are trying to maintain the balance in the world. This Devil would be very cunning and would constantly be devising new ways to control Ichigo.
A new character could be introduced as the main character. For instance, a transfer student who has this hypnotic power. He could be mysterious at first, and as he gets involved with the harem, he gradually reveals his true self and the reasons behind his power.
It could be a rogue fallen angel who is jealous of the godlike SI's power. This fallen angel might see the SI as a threat to their own plans of ruling the underworld or causing chaos between the three factions in the High School DxD world.