Well, you could write a fanfiction where the Messiah in the DxD world has to adapt to the modern and often chaotic world of DxD. For instance, dealing with the different races' cultures and politics. The Messiah could start as an outsider and gradually become an important figure in uniting different groups against a powerful evil force that wants to disrupt the balance in the DxD world. This would involve a lot of character development and exploration of the DxD world's rules and hierarchies.
One of the popular tropes in Persona fanfiction is 'alternate reality'. Writers like to explore what would happen if the events in the game took a different turn. In Highschool DxD, the 'rivalry' trope is common, especially between different devil clans. In a fanfiction that combines both, a trope could be 'culture shock'. A Persona character coming into the Highschool DxD world would experience a huge culture shock due to the different rules, species, and social structures. And also, the 'mentor - mentee' trope could be used. Maybe a powerful character from Persona becomes a mentor to a young devil in Highschool DxD, guiding them through their power - up journey.
One way to blend them creatively is through character crossovers. For example, imagine a character from Persona with the special powers they possess, suddenly finding themselves in the world of Highschool DxD. They could use their Persona powers to deal with the various supernatural situations in the Highschool DxD universe. Another aspect could be the exploration of the different social dynamics. In Persona, there are deep relationships within the group, and in Highschool DxD, there are complex hierarchies among the devils and other beings. By bringing characters from Persona into this world, new relationships and power struggles can be created. Also, the theme of self - discovery, which is prominent in Persona, can be combined with the coming - of - age and power - gaining journey in Highschool DxD.
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.
Issei and his group could be potential allies. They are often on the side of good and protecting the balance in the DxD world. Their strength and determination would make them great companions for the Messiah.
Rias from Highschool DxD is likely to be a popular choice in crossovers. Her status as a powerful devil and her leadership role can make for some exciting scenarios when combined with the Persona 3 universe. Another popular character could be Aigis from Persona 3. Her robotic nature and combat abilities can be quite interesting when pitted against the supernatural beings in Highschool DxD.
Another idea in the crossovers could be the exchange of technology or magic. The Velvet Room from Persona might have some connection to the magic system in Highschool DxD. Maybe the Velvet Room attendant could offer some insights or tools to the characters in Highschool DxD, like a special artifact that helps in controlling their powers better.
In this fanfiction, an interesting idea is the customization of Personas by the Highschool DxD characters. Since in Persona games, you can customize and evolve your Personas, it would be exciting to see how the characters in Highschool DxD would do the same. Maybe they would use their devil powers or unique skills to modify their Personas, creating really powerful and unique combinations for battles and challenges.
Issei is often a very popular character. His pervy nature and his growth as a character in the original Highschool DxD series make him stand out. In the fanfiction with Persona and gamer elements, his ability to adapt to new powers and situations can be really interesting to explore.
In fanfiction, the differences can be even more pronounced. A fanfic might exaggerate the serious and brooding nature of a Persona character when they are put into the more flamboyant and action - packed world of Highschool DxD. Or it could show how a Highschool DxD character's brashness clashes with the more methodical and calculated approach of a Persona character. For example, if a Persona character is trying to strategize in a battle, a Highschool DxD character might just rush in, creating an interesting dynamic.