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.
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.
A heroic knight from a medieval - themed anime. He could be transported to the Highschool DxD world and, due to his chivalrous nature, attract the female characters. He might initially be at odds with the devils but eventually form a bond with the main character and become part of the harem as he adapts to the new world.
One possible aspect to consider in a Naruto and Highschool DxD harem fanfiction is the character interactions. For example, how Naruto's positive and determined personality might mesh with the various characters from Highschool DxD. His ninja skills could also add an interesting dynamic to the story. Maybe he could teach some of the characters self - defense or strategic thinking. Additionally, the harem element could involve Naruto forming close relationships with multiple characters from both universes, but in a non - sleazy way, perhaps based on mutual respect and shared adventures.
One idea could be that the characters from Highschool DxD end up in the Hellsing universe. Maybe Issei and his friends are mistaken for new types of vampires due to their devil powers. They then have to team up with the Hellsing organization to clear their names and also fight against a common enemy that is threatening both the devil and human worlds in this combined universe.
A third idea is that there's a mix - up with the curses in both worlds. Ranma's curse of changing into a girl when splashed with cold water gets combined with some of the magic in Highschool DxD. This creates a whole new set of problems and adventures for him as he tries to figure out how to deal with this strange new situation while also trying to keep his identity a secret from the new characters he meets.
Yes, there are some great ones. You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. Just search for 'Highschool DxD' in the fandom section and then filter through the stories that involve a harem aspect. Some might have Discord communities related to discussing and sharing those fanfictions as well.
One common theme is the development of relationships. Since it's a harem - type story, the main character, usually Issei, forms different kinds of relationships with the female characters. There's often a lot of romantic tension and the exploration of how these relationships might grow over time.
One great Issei harem fanfiction is 'Issei's Ascent'. It really delves into his character development while building his harem in an interesting way. The story has unique plot twists and the interactions between Issei and the members of his harem are well - written.