Luna Lovegood might be an unexpected character to interact with Regulus. Given her unique perspective on magic and the wizarding world, she could offer him a different view on life after his resurrection. Also, Kreacher, the house - elf who was loyal to the Black family, would surely be involved in his story, as he was very attached to Regulus.
Hermione Granger could be another character. She's always interested in knowledge and history, and Regulus' resurrection would be a mystery she'd want to solve. She might study his case, try to figure out the magic behind it, and in the process, they could develop an interesting relationship, with Hermione asking him about the old ways of magic in the Black family.
Well, Narcissa Black (her sister) would surely interact with Bellatrix. They could be shown having sisterly moments, but also Bellatrix influencing Narcissa towards darker ways. And then there's Sirius Black. Even though they are on opposite sides, there could be some tense family - related interactions at Hogwarts. Maybe Sirius tries to convince Bellatrix to turn away from the dark side, but she refuses and they have a big argument in one of the Hogwarts corridors.
Obviously, Harry Potter and Regulus Black are the main ones. Harry as the boy who lived, and Regulus as his guardian, would be central to the story. Their relationship would be explored deeply.
Neville Longbottom might also be a character. Neville has his own connection to the fight against the dark side. He could share his experiences with her and they could bond over their shared desire to break free from the dark legacies of their families. Also, Luna Lovegood could be in the story. Luna's unique perspective on life and magic would be a great contrast to the daughter's more complex family situation, and they could have some very interesting conversations.
Definitely Naruto and the Punisher first. Naruto would be intrigued by the Punisher's methods and would try to understand his motives. It's also possible that Sakura could interact with the Punisher. She's a strong - willed character, and she might try to either stop the Punisher if she thinks his actions are too extreme or help him if she sees the good in his goals. And Jiraiya might also be a character who could cross paths with the Punisher. Jiraiya's worldliness and his own sense of right and wrong could lead to some deep discussions between them.
Naruto and Ruby are very likely to interact. Ruby is the main character in RWBY, and Naruto being the central figure in his own story, their paths would surely cross. Ruby's enthusiasm for combat and her leadership qualities could make for an interesting dynamic with Naruto's determination and his own brand of leadership. Also, Weiss could interact with Naruto. Given Weiss's strict upbringing and her proficiency in combat, her reaction to Naruto's unorthodox ninja techniques would be fascinating.
Sakura from Naruto might interact with Louise from Familar of Zero. Both are strong - willed female characters. They could start off as rivals, competing in areas like magic vs medical ninjutsu, but then gradually become friends and learn from each other's strengths.
Gray is another candidate. Gray's cool and collected demeanor could contrast with Tobi's more mysterious nature. They could get into arguments about the best way to handle a situation, like when facing a powerful enemy. Or, they could find common ground in their skills and work together to protect Fairy Tail from danger.
Death from DarkSiders and Black Widow could have an interesting interaction. Death has seen countless deaths and has a very different perspective on life and death compared to the human Black Widow. Their conversations could be deep and thought - provoking as they work together on a mission.
Iron Man is another candidate. Tony Stark's technology and Naruto's ninja techniques could be a great contrast. They could learn from each other. Stark might be interested in Naruto's chakra - based powers and try to replicate them in some way, while Naruto could be amazed by Stark's high - tech gadgets.
Hermione and Betty would be likely to interact. Hermione's intelligence and thirst for knowledge could pair well with Betty's investigative nature. They could work together to solve a mystery that spans both the wizarding world and Riverdale.