One possible plot twist could be that Harry, under Crowley's influence, starts to question the traditional concepts of good and evil in the wizarding world. He might be exposed to dark magic in a different light, not as something purely evil but as a tool with its own uses. This could lead to conflicts with his friends from Hogwarts who adhere to the more conventional views.
He would probably have a different view on magic. Crowley's knowledge of magic, which may be more on the dark side, would shape Harry's understanding. For example, he might be more willing to use magic for personal gain rather than for the greater good as he was taught in the original story. He could also be more proficient in some types of magic that are considered taboo in the normal wizarding world.
In such a fanfiction, Harry's upbringing by Crowley would likely lead to a very different Hogwarts experience. His magic might be influenced by Crowley's teachings, making it more unorthodox. He might also have a different group of friends or allies, perhaps those on the fringes of the wizarding world who are more in tune with Crowley's ways.
A plot twist could be related to Fem Harry Potter's magical abilities. Since she is raised by Crowley, she might develop some unique magical skills that are a blend of wizarding magic and Crowley's own brand of 'power'. This could make her a target for various groups in the wizarding world who either want to use her for their own gains or are afraid of her new - found powers. Also, Crowley might have some enemies in the wizarding world that he has never revealed to Fem Harry Potter. When these enemies discover her existence, they target her to get back at Crowley, putting her in great danger and forcing her to rely on skills and knowledge she has learned from Crowley to survive.
The biggest deviation would be in Harry's character. In the original, Harry is inherently good - hearted. But with Crowley as his mentor, he might be more morally grey. His decisions would be based more on self - interest rather than the common good.
Fem Harry Potter's character would develop a greater sense of independence and self - reliance. Crowley is not the type to coddle, so she would have to learn to fend for herself from a young age. This could lead to her being more confident in her own abilities, but also more solitary. She may not feel the need for the approval of others as much as the original Harry Potter. Also, her moral compass might be more flexible. Since Crowley doesn't adhere to strict good - vs - evil concepts, she would be more likely to question the black - and - white view of magic that is prevalent in the wizarding world.
In fanfics, the 'weight' could refer to the significance or importance of Aziraphale and Crowley. It might be about how much they matter to the overall plot or to each other. For example, a fanfic could focus on how their relationship is the 'weight' that holds the story together during a time of great upheaval in the Good Omens universe.
I'm not entirely sure as I haven't read this specific fanfic. But generally, if it's 'Harry raised by Gai', it might be a story where a character named Gai takes on the role of raising Harry in an alternate universe or fan - created scenario.
I would recommend 'Aziraphale and Crowley's Adventures in London'. This fanfic is full of fun scenarios where the two characters explore different parts of London. It shows their friendship in a very light - hearted way. The descriptions of the city are also quite vivid, which adds to the overall charm of the story. It's a great starting point as it doesn't have a very complex plot and you can easily get a feel for how the two characters interact.
I really like 'Crowley and Aziraphale's Celestial Adventures'. It has a lot of humor in it. For example, there are some really funny misunderstandings between Crowley and Aziraphale that add a light - hearted touch to the story. It also delves into their backstories in a creative manner, which helps to flesh out their characters even more.