In some fan fictions, there could be a romantic sub - plot between Harry and someone in Bill's circle. Bill, seeing the potential for this relationship to grow, decides to save Harry from the Dursleys. He might think that by getting Harry out of that toxic environment, Harry will be more open to love. Also, Bill's own romantic experiences might have made him more aware of the importance of a loving and supportive environment, and he wants to provide that for Harry.
The romance could be that Bill has a love interest who encourages him to save Harry. Maybe his girlfriend or wife has a soft spot for Harry and convinces Bill to step in.
In some fan fictions, Bill might use his experience and magical skills. He could arrive at the Dursleys' place under the guise of some official wizarding business. Once there, he realizes the horrible situation Harry is in. Maybe he convinces the Dursleys that it's in their best interest to let Harry go with him, using a bit of magic to persuade them subtly.
The depression might be a hurdle that Bill has to overcome in his romantic journey. It could make him doubt himself and his worthiness of love. Maybe at first, he is hesitant to pursue a relationship because of his low mood.
There's also the idea of envy. Voldemort might envy the normal life that Harry could have had if not for the wizarding world's interference. By saving Harry from the Dursleys, he is both taking Harry away from a life he could never have himself and also trying to create a new situation where he can be in control of Harry's life, rather than the Dursleys or the Order of the Phoenix.
Well, in some fanfictions, Bill might use his magical skills and knowledge. He could cast spells to create distractions for the Dursleys, allowing Harry to escape unnoticed. For example, he might make a flock of magical birds fly into the Dursley's yard, causing chaos while he smuggles Harry out.
Maybe Severus discovers some dark magic plot that involves the Dursleys harming Harry in a more serious way. So, he intervenes just in time. He could use his skills in potions and magic to subdue the Dursleys if they resist, and then take Harry to a safe place, like his own secret hideout or a friendly wizard's home.
It could be Voldemort. He might have influenced the Dursleys in some way to harm Harry, and Severus has to save Harry from Voldemort's long - reaching plans. For example, Voldemort could have placed a curse on the Dursleys' house that would eventually harm Harry, and Severus discovers it just in time.
Maybe Percy uses his magic skills to create a distraction. He could conjure up some strange creatures outside the Dursleys' house. The Dursleys, being afraid of all things magical, would be distracted long enough for Percy to sneak in and get Harry out.
In the fan fiction, Severus could start to develop a sense of responsibility towards Harry. At first, he may have saved Harry out of a sense of duty to Lily Potter, but as time goes on, he realizes that Harry has his own worth. This could make him more open to forming a bond with Harry, which is a huge change for his character who has been so closed - off for so long. He may also start to question some of his long - held beliefs about the Gryffindors, especially as he sees Harry's courage in the face of his difficult upbringing at the Dursleys.
Obviously, Severus Snape and Harry Potter are the main characters. Severus is the one doing the saving, and Harry is the one being saved from the Dursleys. But there could also be other characters involved. For example, Dumbledore might make an appearance. He could either support Severus's actions or be a bit wary of them. Petunia Dursley could also be a significant character as she is part of the reason Harry needs to be saved from that household.
In some fan fictions, Severus might save Harry from the Dursleys because he realizes that Harry is the key to defeating Voldemort in the long run. He might see that the Dursleys' treatment is making Harry weak and less likely to be able to face Voldemort. So, he intervenes, maybe by intimidating the Dursleys into treating Harry better and providing him with proper education about the wizarding world. He could also give Harry some potions to boost his health and magical abilities that have been stunted by the Dursleys' neglect.