In fanfiction, the power system of Highschool DxD is often based on devil powers, sacred gears, and the power of dragons. It's very supernatural. Akame ga Kill's power system, on the other hand, is more about combat skills, unique weapons, and physical strength. When combined in fanfiction, it can create a contrast where the magic - based powers of Highschool DxD can either overpower or be challenged by the more down - to - earth combat abilities of Akame ga Kill.
In Harry Potter, the power system is mainly based on magic. Wizards and witches are born with the ability to use magic, and they learn spells and potions at Hogwarts. In Highschool DxD, the power system is centered around Sacred Gears, which are artifacts that give the users various abilities. Magic in Harry Potter seems more about knowledge and practice, while Sacred Gears in DxD are more like inborn or found items of power.
Another approach is to consider the energy sources. In Prototype, there's the virus as a source of power. In Highschool DxD, there's magic and spiritual energy. Maybe the virus could be a corrupted form of magic in the fanfiction. Characters could then try to either purify or control this 'viral magic' and use it in combination with their existing powers from the Highschool DxD universe.
One interesting crossover could be the characters from Highschool DxD using their unique powers in the world of Akame ga Kill. For example, Issei's Boosted Gear could potentially give him an edge against the powerful foes in Akame ga Kill. Another idea could be the assassins from Akame ga Kill teaming up with the devils from Highschool DxD to take on a common enemy that threatens both their worlds.
One idea could be to have characters from both universes meet in a strange, new dimension. For example, Tatsumi from Akame ga Kill could end up in the Highschool DxD world and have to adapt to the different rules and powers there. He might form an alliance with Issei to fight a common enemy that threatens both their worlds.
Start with a mystery that ties both universes together. For instance, a powerful artifact has gone missing from Akame ga Kill's world and has ended up in Highschool DxD's world. Characters from both sides must work together to find it. This creates an immediate goal and a reason for them to interact.
In fanfiction, the power systems are often compared in terms of raw strength. For instance, the Quirks in My Hero Academia can range from super strength to more esoteric abilities like the ability to create portals. In Fairy Tail, magic is more diverse, including elemental magic like fire and ice, and also some more unique forms like celestial spirit magic. In crossovers, it's fascinating to see which power would prevail in a battle.
Well, let's think about it this way. The power levels in both worlds are different. In Naruto, there are different levels of ninja skills and chakra control. In Highschool DxD, there are different ranks of devils and angels with their own powers. When these two worlds collide in fanfiction, we might see a devil using their power to try and disrupt Naruto's chakra network, which could lead to Naruto having to find new ways to use his chakra. On the other hand, Naruto's chakra - based attacks could potentially break through some of the magical defenses in Highschool DxD.
In some fanfictions, the merge happens when a powerful artifact from either universe is activated. This could be a Kamen Rider's belt or a powerful relic from Highschool DxD. When this happens, it creates a rift that pulls elements from one universe into the other. The characters then have to deal with the new and unexpected situations. For instance, the devils in DxD might be intrigued by the Kamen Rider technology and try to harness it for their own purposes.
In a fanfiction, the power systems could combine in interesting ways. For instance, the chakra in Naruto could be related to the magic in Highschool DxD. Maybe a character could use chakra to enhance their magical abilities. Or, the devil's powers in Highschool DxD could be used to boost the physical abilities of a Naruto character, similar to how the Nine - Tails chakra enhances Naruto's strength.
In these fanfictions, the power systems can interact in some really cool ways. Naruto's chakra system is very different from the devil powers in Highschool DxD. But when combined, you could have a situation where Naruto's chakra is used to enhance a devil's attack. In Rosario Vampire, the vampire powers are more about physical abilities and some magic. They could interact with Naruto's ninjutsu in a way that creates new forms of combat, like a vampire using Naruto's shadow clone jutsu to create multiple decoys.