One interesting storyline could be Alucard forming an unexpected alliance with the main characters in High School DxD. Maybe he uses his unique powers to help them in a battle against a common enemy.
Rias Gremory is also quite popular. When she is paired with Alucard in fanfiction, their different personalities and powers create interesting scenarios. She is a strong and intelligent character, and her relationship with Alucard can range from friendly cooperation to a more complex, power - based relationship.
Maybe a story where Alucard, Naruto, and a character from DxD end up in a strange new dimension. Naruto uses his ninja skills to scout the area, Alucard provides protection with his vampiric abilities, and the DxD character uses their own powers to try and find a way back home. Along the way, they have to face new and dangerous enemies that are a mix of all three universes.
It's hard to say for sure, but Alucard has the potential to be extremely powerful. His vampiric abilities like regeneration, immortality, and the ability to summon his army of familiars give him a huge edge. He can take a lot of damage and keep coming back. In a fight against Naruto's ninjutsu or the various powerful beings in the DxD world, he could be a force to be reckoned with.
The characters in DxD would be quite shocked at first. Ichigo's appearance and his powers are so different from what they are used to. For example, Rias Gremory would probably be intrigued by Ichigo. She might try to form an alliance with him, seeing his potential as a powerful ally. Issei, being the main character in DxD, would likely be both jealous and in awe of Ichigo's abilities, and might challenge him at first but then learn from him over time.
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.
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.
Alucard is significant as he represents power and mystery. His long existence gives him a depth that is alluring in the fanfiction. Seras, on the other hand, is important as she is the new - comer to the vampire world. She is a bridge between the human world and the vampire world, which makes her character development interesting. Together, they create a dynamic where old and new vampire cultures collide and interact.