One possible plot is that Gilgamesh views the High School DxD setting as beneath him at first. But as he gets to know the characters like Issei and Rias, he starts to see the value in their struggles. He could then use his vast resources and power to help them in their fight against stronger enemies, all while maintaining his haughty and kingly demeanor.
His personality would be a mix of pride and a sense of superiority. He'd walk around the school as if he owns the place. However, he might also have a sense of honor. For example, if he made a promise to a character in the DxD world, he would keep it, but in his own haughty way. He might also be somewhat dismissive of the school's rules, believing that he is above them.
The plot could involve Ash making new friends in the High School DxD setting. He might befriend some of the devils like Rias Gremory. His kind-hearted nature from the Pokémon world could be a contrast to the more complex and often self - interested characters in High School DxD. He could introduce them to Pokémon and they could in turn teach him about their magic and powers.
Well, perhaps she is there to find a powerful artifact hidden in the school. This would bring her into conflict with the different groups in the school, like the devils who also want the artifact. And as she searches, she has to deal with the social and power structures of the school which are very different from her own world.
Well, it could be about Ichigo having some super - transcendent powers that he has to hide in the High School DxD world. Maybe he's a new student there and his powers start to disrupt the normal balance of the supernatural elements in the school.
In a Gohan in High School DxD fanfiction, Gohan could be a special exchange student. He has to balance his studies and his new life in this strange world filled with devils and angels. He might start a martial arts club in the school, which becomes popular. But his real purpose could be to find a way to bring peace between the different factions, using his Saiyan abilities and kind heart.
Perhaps the story could revolve around a conflict between the Ghost Rider's sense of justice and the more complex moral and power dynamics in the High School DxD universe. For example, he might not agree with how the devils in the school operate, and this could lead to a series of confrontations and alliances as he tries to find his place within the school while still adhering to his own code.
Well, it could be that Naruto, as a Nephilim, enters the High School DxD world. He might use his ninja skills along with his new Nephilim powers. Maybe he has to deal with the devils and angels in the school setting, while also trying to keep his identity a secret. He could form new alliances and rivalries within the school, all while facing challenges that are unique to this crossover world.
Perhaps in this fanfiction, Naruto and Kushina are reincarnated in the High School DxD world. They have to adapt to the new school life there. Naruto would surely stand out with his determination and fighting abilities. Kushina would be a motherly figure to some of the students who lack that kind of support. They both could change the balance of power between the different factions in the world.
One possible plot could be Sans getting involved in the supernatural battles in the High School DxD universe. Maybe he uses his unique abilities from his original game to help out the main characters in a fight against a powerful enemy. For example, his bone attacks could be a useful addition to the combat arsenal in that world.
A plot could involve the Annihilation Maker creating a device that can enhance or steal the powers of the characters in High School DxD. This device falls into the wrong hands, like a group of rogue exorcists who want to use it to purify all the devils and fallen angels. The main characters then have to race against time to retrieve the device from the exorcists. Along the way, they face many challenges and battles, and the Annihilation Maker has to use their skills to reverse the effects of the device if it has already been used on some of the characters.