Seras could be another main character. Her transformation and growth in the Hellsing series make her an interesting character to pair with Hans. They could have a mentor - mentee type of relationship, where Hans helps Seras further develop her powers or understand the new world she's in.
Obviously, Hans would be one of the main characters. He would bring his own set of characteristics and abilities into the Hellsing universe. Alucard is also likely to be a main character as he is one of the central figures in Hellsing. His overpowered nature and complex personality would create interesting interactions with Hans.
One interesting concept could be the exploration of different power dynamics. For example, how Hans' unique abilities might interact with the supernatural elements in Hellsing. Maybe his skills could be enhanced or modified in the Hellsing universe, leading to new and unexpected storylines.
Obviously, Gaster himself is likely to be a main character. His mysterious nature makes him a central focus as readers are eager to learn more about him. Another potential main character could be Frisk. Their ability to reset and influence the game's world could be very interesting when combined with Gaster's enigma. They could either be allies or have some sort of conflict due to their different understandings of the world.
Sir Integra from Hellsing could interact with the higher - ups in the devil society of Highschool DxD. They would likely have diplomatic meetings to discuss how to deal with threats that cross over between their two worlds. Integra would be firm and no - nonsense, which could either earn her respect or cause some friction with the devils who are used to a more complex power - play based society.
Jackie Estacado from 'the darkness' is an obvious choice. His connection to the Darkness power makes him a central figure and his skills and personality can drive the story forward in a crossover.
Cole MacGrath from 'Infamous' could be a main character. His electric powers would be really cool to see in the wild west. He could use his powers to stop outlaws or protect a town from bandits.
One potential crossover character could be Harry Potter. Imagine Naruto, with his immortality, meeting Harry in the wizarding world. Naruto could teach Harry some ninja skills, and they could fight together against Voldemort. Another could be Superman. The contrast between Superman's superpowers and Naruto's ninja skills, along with Naruto's immortality, would make for an interesting story.
Another potential character is Harry Potter. His wizarding skills and knowledge of magic could be a contrast to Naruto's ninja abilities. Imagine Naruto in Hogwarts, learning about magic while also showing off his ninja moves. They could also face a threat together, like a dark wizard who is trying to use both magic and chakra for evil purposes.
Maybe Naruto from Naruto could be a crossover character. His ninja skills and Ichigo's combat abilities could create some unique fight scenes. Also, their different ideologies about friendship and power could lead to some great character interactions.
Naruto himself could be a great crossover character. Since Kakashi is his mentor in the original series, seeing them interact when Kakashi is a kid could be really interesting. They could have a different kind of dynamic, with Naruto perhaps being the more experienced one in some aspects. Another potential character is Goku from Dragon Ball. The contrast between Kakashi's ninja skills and Goku's martial arts and superpowers would create a really exciting crossover.
Solid Snake is an obvious choice. Since they are from the same general franchise, their encounter could be filled with a lot of drama and action. They could have different views on how to handle certain situations, which would create great story tension.