Often through character design. In some fanfics, a character from Highschool DxD might be given an appearance or ability inspired by a Warhammer character. For example, a devil in DxD could have armor similar to a Space Marine's.
Magic systems can also be combined. The unique magic in Warhammer, whether it's the sorcery of the Empire or the psychic powers in 40K, can be blended with the magic used in Highschool DxD. This can lead to some really creative spellcasting and power - using scenarios. For example, a character might combine a DxD spell with a Warhammer - style psychic blast to defeat an enemy.
Well, you could have the OC Archangel fall in love with one of the human characters. Maybe it starts as a simple attraction because the human has a kind heart or a strong will. For example, if the human is always standing up for the weak in the school, the OC Archangel might be drawn to that. Then, as they spend more time together, their feelings grow deeper.
Multiplayer aspects can be added too. Imagine the OC starts an in - universe gaming club where characters from different species in Highschool Dxd can compete against each other in various games. This could lead to some interesting rivalries and friendships, like a devil and an angel teaming up to win a game against a group of fallen angels.
You can start by making martial arts a part of the OC's backstory. Maybe they grew up in a dojo or were trained by a martial arts master. This gives a reason for their skills.
Another one to check out is 'DxD and Warhammer: A Crossover of Power'. In this fanfiction, the author takes the opportunity to create new power - up scenarios for the Highschool DxD characters by introducing elements from Warhammer. For instance, a character might gain a weapon inspired by a Warhammer artifact, which then changes the dynamics of battles within the DxD universe. It's a really exciting read for fans of both franchises.
In a Highschool DxD OC and Sharingan fanfiction, you can incorporate the Sharingan powers in the story by making it a source of mystery and envy among the other characters. The OC could use the Sharingan's ability to enter the minds of others (a more advanced form) to gather information about the devils' plans. This would make the OC a valuable asset to their group. Also, the Sharingan could have a connection to some ancient prophecy in the Highschool DxD world, which would add more depth to the story and make the powers more significant in the overall plot.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
One way to incorporate different Saiyan forms is to base the transformation on the power - up systems in Highschool DxD. In DxD, characters often gain power through training, contracts, or special items. You could make it so that a character has to collect a certain number of dragon balls (in a fanfiction - inspired crossover way) to unlock a new Saiyan form. Or they might need to make a pact with a powerful being in the DxD world to be able to access Super Saiyan 3. This way, you blend the two universes' power - up mechanics in an interesting and unique way.
Fem Issei can influence the story's themes. In Highschool DxD, themes like masculinity and male - centric power struggles are common. With Fem Issei, themes of femininity, female empowerment, and different forms of leadership can emerge. This can completely transform the tone of the fanfiction, making it more about female agency and how she challenges the existing norms of the devil, angel, and fallen angel societies in the story.
The godlike oc can greatly impact the plot by being a game - changer. It might have abilities that can easily solve problems that were difficult for the original characters, thus changing the direction of the story. For example, if there was a big battle coming up, the godlike oc could use its powers to either prevent the battle or completely dominate it in a way that was unexpected.
In many fanfictions, Issei's character develops in terms of power. He starts as a relatively weak character and gradually becomes stronger through training and various challenges. For example, he might learn new techniques from a mysterious mentor or gain new powers from a magical artifact.