Well, aside from the obvious ones like Alucard, Integra, and Seras, Bella might also interact with Walter. Walter is an old and experienced member of the Hellsing staff. He could be a mentor figure to Bella, teaching her about the history and secrets of the Hellsing Organization. Another character could be the Major from the Millennium Organization. If there's a crossover or a story where the two organizations clash, Bella would surely cross paths with him. His crazy and unpredictable nature would make for some interesting interactions with Bella.
Definitely Aro. He is the leader of the Volturi, so he will have a lot to do with Bella.
Caroline Forbes is another character. As a vampire herself in Mystic Falls, she could either befriend Bella or be jealous of her presence. Caroline is known for her strong personality and her relationships within the vampire community.
Stiles is another key character. He's smart and curious, and he would probably be intrigued by Bella's arrival. They could team up to figure out some of the strange happenings in town. Also, Lydia, with her intelligence and unique abilities, could form an interesting relationship with Bella, maybe as a confidante or a rival in some situations.
Edward would definitely interact differently. If Bella goes back in time and changes her initial approach, Edward might be more or less intrigued by her. For example, if she's more confident, he might be even more drawn to her than in the original story.
It could be her best friend. Her best friend might be the one who accidentally discovers Bella's secret in the fanfiction. And then they have to deal with the situation together, whether it's keeping the secret or helping Bella face whatever the secret entails.
Edward Cullen would likely be a main character. His reaction to Bella's situation, whether it's concern for her well - being or struggle with his vampiric instincts around her period blood, could be a major part of the story.
She might meet Aro, one of the leaders of the Volturi.
She might meet her younger self. It would be an odd yet fascinating encounter, as she could see how she used to be and compare it to who she is now.
There is also a possibility that a lone vampire, who has a sense of justice or who has been secretly observing Bella and has grown fond of her, decides to take on the Volturi and rescue her. This vampire could have some special abilities or knowledge that gives them an edge against the powerful Volturi.
She could interact with Elena. Since Elena is a central character in Mystic Falls, their paths are likely to cross. They might start as friends or rivals, depending on the story.