One possible scenario could be that Remus has come to check on Harry secretly. Maybe he wants to make sure Harry is okay and see if he needs any help with his magic studies in secret, away from the Dursleys' watchful and often unkind eyes.
One possibility is that Remus meets the Dursleys when he is on some sort of mission related to Harry. He could be there to assess the safety of Harry's living situation. He might find the Dursleys' treatment of Harry unacceptable and try to subtly change things. He could offer to help around the house in a very civilized way, making the Dursleys a bit confused as they are not used to such kindness from the wizarding world. This could lead to a series of events where the Dursleys start to question their own views about wizards and witches.
Remus might interact with the Dursleys in a very calm and collected manner. He would probably be polite, despite their rudeness.
One possible scenario could be that the Dursleys, out of their long - held hatred and jealousy, manage to poison Harry's food. In the fanfiction world, they might find a way to obtain a powerful and undetectable poison. Another could be that they stage an 'accident' like a fire in his room while he's asleep. They could block his escape routes and let the fire consume him.
One possible scenario could be that Harry uses his magic skills in a creative way to escape. Maybe he creates a diversion with a spell to distract the Dursleys and then slips away unnoticed. Another could be that he gets help from an unexpected ally, like a friendly magical creature that guides him out of the house.
One possible scenario could be that Harry discovers a hidden magical passageway in his tiny bedroom at the Dursleys'. It could be a secret portal created by a well - meaning wizard long ago to help any oppressed magical being in the Muggle world. Harry stumbles upon it while looking for a lost sock or something mundane. Another scenario might be that a group of friendly magical creatures, like house - elves, conspire to help him escape. They create distractions around the Dursley house, like making the pots and pans fly around and creating chaos, which allows Harry to slip away unnoticed.
One scenario could be that Bill uses his charm to charm the Dursleys into letting Harry go. He might be really smooth - talking and make them believe that it's in their best interest to send Harry away with him. Maybe he offers them some sort of compensation in the wizarding world, like a magical object that will keep their house clean forever.
One possible scenario could be that a powerful wizard or witch, perhaps someone who knew James and Lily Potter well, comes to rescue baby Harry. They might use magic to bypass the Dursleys' defenses and take Harry away to a safer place, like a hidden wizarding community.
The Dursleys are a rather uptight and narrow - minded family. In this fanfiction scenario, they could be extremely judgmental of Harry's girlfriend. They might look down on her if she doesn't fit their idea of what a proper person should be like. They may also be secretly afraid that she will expose their mistreatment of Harry, so they could act very defensively during her visit.
Well, it could be that the Dursleys are still as cold as ever at first. But Harry, having grown so much during the war, doesn't let their attitude affect him. Instead, he just calmly states his new life and how he has moved on from their influence. Maybe this makes the Dursleys start to see him in a new light and gradually change their view of him.
Well, Sirius might be shocked by the Dursleys' treatment of Harry. He could try to rescue Harry from their mean ways.