Well, she might struggle with her newfound vampiric nature. Maybe she would try to resist the urge to feed on humans, like Angel did. She could also find herself at odds with Buffy, who would be torn between protecting her sister and fighting the vampire she has become.
I think there are several such fanfictions out there. You might find one on popular fanfiction websites like Archive of Our Own or FanFiction.net. But specific titles could vary.
It could change the whole dynamic. Buffy would have a personal stake in dealing with vampires in a different way now. Instead of just slaying them all, she might try to find a cure for Dawn. This could lead to her making deals with other vampires or searching for ancient magic.
One interesting plotline could be Bella's struggle to control her new vampire instincts while still maintaining her relationships with her human friends and family. For example, she might have a hard time being around them without revealing her new nature.
Her relationships with her human friends will surely be affected. They might be scared of her at first, or she might be afraid of hurting them. So she may start to distance herself from them.
Well, she might struggle with her newfound vampire nature. Maybe she has a hard time controlling her thirst for blood and accidentally attacks someone she cares about.
A must - read vampire Buffy fanfiction is 'Shadows of the Night'. This one shows Buffy dealing with a new breed of vampires that are much more cunning and powerful. The author does an excellent job of building the tension throughout the story.
Edward is almost always there. Since he's such a central figure in Bella's life as a human and vampire in the original story, he would likely play a big role in the fanfiction too. He might be the one guiding Bella through her new vampire life.
In a fanfic where Harry gets turned into a vampire, one thing that could occur is that he would be shunned by the wizarding community at first. They would see him as a threat. But he could also use his vampire powers to his advantage in fighting the dark forces. He might be able to use his enhanced senses to detect danger earlier. Also, his immortality could change his perspective on life and his long - term goals. He could become more isolated as he outlives his friends, or he could try to find a way to reverse the transformation.
Spike is often paired with Buffy in this type of fanfiction. Since they already had a complex relationship when Buffy was a slayer, it becomes even more interesting when she becomes a vampire. Angel is also a common pairing. Their past relationship and his own experience as a vampire with a soul add a lot of depth to the story.