A crossover with 'One Piece' is also quite common. Just imagine the chaos that would ensue if the Straw Hat crew landed in the world of Highschool DxD. Luffy's carefree attitude and his powerful devil fruit abilities would be a great contrast to the more serious and power - hungry devils in DxD. And of course, his nakama would surely get involved in the harem situation in some way. Maybe Nami would be interested in the treasures that the devils possess, or Sanji would be chasing after the female characters in DxD.
One idea could be to cross it with a magical girl anime. The main character from Highschool DxD could find himself in a world full of magical girls and form a harem with them. They could fight together against common enemies while also dealing with the typical harem situations like jealousy and misunderstandings.
One popular plotline is the self - insert character gradually building relationships with different members of the harem. For example, they might start by befriending a few characters and then over time, romantic feelings develop. Another is the self - insert getting involved in the battles and power struggles in the Highschool DxD universe while also dealing with the harem dynamics. It could be that they have to prove themselves worthy to be part of the group and in the process, win the hearts of the harem members.
One popular crossover might be the exploration of the unique powers. For example, Naruto's jutsu in the Highschool DxD world could create interesting scenarios. His shadow clone jutsu could be used in battles against the devils or fallen angels in the Highschool DxD setting. Another aspect could be the character relationships. Naruto's friendly and determined nature could form unique bonds with the characters in Highschool DxD like Issei. This could lead to new storylines where they team up to face common enemies or help each other grow in power.
First, you need to know the source materials well. Thoroughly understand the characters, settings, and rules in both Highschool DxD and the other series you want to crossover with. For example, if it's a crossover with 'Tokyo Ghoul', know how the ghouls' powers work and how they would interact with the devils in DxD. Second, focus on character development. Don't just throw the characters together randomly. Let their interactions change and develop them. For instance, if Issei meets Kaneki, they could learn from each other's experiences of being 'different' in their respective worlds.
One interesting crossover could be the characters from Naruto learning about the devil and angel society in Highschool DxD. For example, Naruto's ninjutsu skills could be a shock to the characters in Highschool DxD. He might use his shadow clone jutsu in a battle against a devil or an angel, which would completely change the dynamic of the fight. Another aspect could be the sharing of values. Naruto's strong belief in friendship and never - giving - up attitude could inspire some of the more self - centered characters in Highschool DxD.
Well, in some fanfictions, Godzilla's presence in the Highschool DxD universe might disrupt the power balance. For example, Godzilla's atomic breath could be a huge threat to the various factions in Highschool DxD. Some fanfictions might explore how the characters adapt to this new and extremely powerful entity. They might try to find a way to communicate with Godzilla, like using magic spells or special artifacts to bridge the communication gap between them and the giant lizard.
A cool crossover could involve the power systems of both. Maybe Sasuke from Naruto tries to master some of the demonic powers in Highschool DxD. He might see it as a way to gain more strength, similar to how he sought power in the Naruto series. This could lead to some intense battles and power - up moments as he tries to combine his Sharingan abilities with newfound demonic powers.
In 'Persona 3 and Highschool DxD fanfiction', a cool crossover might involve the social links in Persona 3 being related to the relationships in Highschool DxD. For instance, the friendship between the characters in Persona 3 could be paralleled with the bonds of loyalty among the devils in Highschool DxD. Also, the Velvet Room in Persona 3 could have some sort of connection to the magic and power systems in Highschool DxD. It could be that the Velvet Room attendant has some knowledge about the devil fruits or the sacred gears in Highschool DxD.
A really fascinating aspect of crossovers in this type of fanfiction is the integration of storylines. Imagine the Akatsuki from Naruto getting involved in the power struggles in the Highschool DxD world. They could be after the Sacred Gears, thinking they are some kind of super - powerful ninja tools. This would completely disrupt the balance in the Highschool DxD universe and lead to all kinds of new plot developments as the characters from both series have to deal with this new threat.
One interesting crossover could be the characters from Highschool DxD using Ben's Omnitrix. Imagine Issei using the different alien forms to fight his battles in the DxD universe. It would add a whole new level of power and strategy.