There's a fanfiction where Issei is a dragon slayer and he teams up with some unexpected characters from the Highschool DxD universe. The story is called 'Issei's Dragon Slaying Adventure'. It delves into his relationships with other characters in a new light as he takes on this new role. He has to face not only the usual enemies but also some who are after his newfound powers.
A common theme is power. The dragon slayer often has to deal with gaining and controlling their power in the Highschool DxD universe. Another theme is friendship, as they interact with the characters from the original series.
Issei would probably be the first to want to fight Thanos. He's always eager to prove himself and protect his friends. He might initially underestimate Thanos due to his own growing power and confidence, but soon realize how powerful Thanos really is. Rias would be more cautious. She would likely try to gather information about Thanos first and then formulate a plan to either fight or negotiate with him.
Issei, being the hot - blooded character he is, would probably be excited to meet someone as powerful as Naruto. He might challenge Naruto to a friendly spar at first, hoping to learn from him and test his own strength. And as he gets to know Naruto better, he could start to see him as a brother - in - arms, especially if they face common enemies together.
The popularity of Highschool DxD Issei Dragon Slayer fanfiction often lies in the power - up aspect. Fans love to see Issei gain new and exciting abilities like dragon slaying powers. It makes him seem more heroic and gives him an edge in the battles within the story.
Start with their backstory. Decide where they came from, how they became a dragon slayer. Were they trained by a master? Or did they have a personal vendetta against dragons? This will shape their personality.
One idea could be an OC who is a dragon slayer but has a unique power that sets them apart. For example, they can absorb the powers of the dragons they slay instead of just killing them. This could lead to all kinds of new storylines as they become more powerful and are hunted by both good and bad forces in the Highschool DxD universe.
Power struggles are very common. Since the original has a lot of characters with different powers, fanfictions often explore who is the strongest and how power can change relationships. For example, a demon might try to overthrow a more powerful one.
Rias would be devastated. Issei was not only her servant but also someone she had deep feelings for. She would likely go into a rage at first, vowing to find and punish whoever was responsible. Akeno would be heartbroken as well. She and Issei had a special bond, and she would be in mourning for a long time. Kiba might feel a sense of guilt, wondering if he could have done more to protect Issei. Asia would be in shock and constantly pray for Issei's soul, hoping for some kind of miracle.
Issei would probably be both scared and excited. He's used to dealing with powerful beings, but the Ghost Rider is on a whole different level. He might initially try to fight him out of bravado, but soon realize he's outmatched. Rias would be more cautious. She would see the Ghost Rider as a potential threat to her peerage and would try to figure out his motives before taking any action.