If we think about the relationships in a Bleach and Highschool DxD crossover. The devils in Highschool DxD are used to a certain hierarchy and power play. When they meet the Soul Reapers, who also have a strict hierarchy, there could be clashes. For instance, Rias and Byakuya might initially have a tense relationship. Rias is used to being in charge in her own way, and Byakuya is a proud captain. But as they see each other's sense of justice and the need to protect their respective worlds, they could start to work together. And for the students in Highschool DxD, they might look up to the more experienced Soul Reapers. The Soul Reapers, in turn, might be amused by the more carefree and hormonal nature of the Highschool DxD students. This could lead to a mentoring - like relationship in some cases.
One possible plot twist is that the hogyoku from Bleach somehow ends up in the hands of a character in Highschool DxD. This could lead to a character attempting to use it for their own gain, perhaps to gain ultimate power over both the devils and the angels. Another interesting plot twist is that a character from Highschool DxD gets reincarnated as a Soul Reaper in Bleach. They would have to learn the ways of the Soul Reapers while still retaining some of their previous abilities and memories. This could create some really interesting character development. A third plot twist could be that the balance between the three factions in Highschool DxD (devils, angels, and fallen angels) is disrupted by the arrival of Hollows from Bleach. The factions would have to figure out how to deal with this new threat, and it could lead to some unexpected alliances.
Well, Issei's Boosted Gear could potentially combine with Ichigo's Zangetsu. The power - amplification aspect of the Boosted Gear might enhance Zangetsu's cutting ability. Imagine Issei lending his power to Ichigo during a tough fight against a powerful Arrancar or something.
One interesting plot could be the characters from Highschool DxD somehow ending up in the Bleach universe. Maybe Issei and his friends are mistaken for Hollows due to their demonic energy and are hunted by Soul Reapers. They then have to prove their innocence while also learning about the complex spiritual world of Bleach.
The character crossovers are a big part. Imagine Ichigo from Bleach meeting Rias from High School DxD. Their different personalities and goals make for really engaging storylines. Also, the fanfic writers can explore how the characters from one universe would react to the situations and rules of the other universe. For example, how would a Bleach character deal with the peerage system in High School DxD? It opens up a whole new world of possibilities for storytelling that makes these crossovers so interesting.
Another plot could involve the Quincy from Bleach. Uryu could be mistaken for a Devil by some of the Highschool DxD characters because of his unique powers. This misunderstanding could lead to a conflict at first, but then they realize they are all on the same side against a greater threat, like a powerful fallen angel who is trying to start a war between the Hollows, Devils and Quincies.
Ichigo and Rias would be a very interesting pairing. Ichigo is a powerful and determined fighter, and Rias is a strong - willed Devil. Their personalities could clash at first, but then they might find common ground in their leadership roles and their desire to protect their respective groups.
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.
One common theme is crossover battles. Writers often like to imagine how the characters from Bleach would fare against those from Highschool DxD in combat. For example, Ichigo might be pitted against Issei.