A possible plot is that Rain is initially controlled by the Dark Gundam but gradually regains his consciousness. As he does, he has to struggle with the memories of the actions he was forced to do while under its control. He then decides to use his knowledge of the Dark Gundam's workings to find a weakness and destroy it. This could involve a lot of internal conflict as he battles his own guilt and the external threat of the Dark Gundam.
Perhaps the story could focus on the relationship between Rain and the Dark Gundam. Rain could be the only one who can understand the Dark Gundam's true motives, which are not entirely evil but rather misdirected. He then tries to change the Dark Gundam instead of destroying it. This would be a unique take as it goes against the typical 'good vs evil' narrative and explores the idea of redemption for a seemingly evil entity.
Rain is the key character here. He's the one in the difficult situation of being shackled to the Dark Gundam. The Dark Gundam is not just a machine but can be considered a character with its own motives and powers. Additionally, there could be a mentor - like figure for Rain. Maybe an older, more experienced pilot who has some knowledge about the Dark Gundam and tries to guide Rain in his struggle. There could also be antagonists other than the Dark Gundam, such as those who oppose Rain's attempts to free himself for their own self - serving reasons.
The plot could involve the Dark Gundam capturing Rain as a way to lure in other characters. Since Rain is likely to have friends or allies, the Dark Gundam could be using him as bait. This would then lead to a rescue mission by those who care about Rain, with lots of action and drama as they try to outwit the Dark Gundam.
The rain could be related to the Dark Gundam itself. Maybe the presence of the Dark Gundam somehow affects the weather, and the rain is a result of its dark power. This could create an interesting plot point where characters have to deal with not only the Gundam but also the environmental changes it brings, like the constant rain.
One idea could be that Sonia discovers a hidden power within the Gundam that only she can unlock. This power could be the key to saving their world from a new, powerful enemy.
Perhaps a story where a builder discovers a hidden ability in a common Gundam model during a build. This new ability then changes the way battles are fought in the Gundam Build Fighters tournaments.
One idea could be a story about a young ninja from the Rain Village who discovers a hidden power within himself. He then has to keep it a secret while trying to protect his village from outside threats. Maybe he has the ability to control the rain in unique ways, like using it as a shield or a weapon.
One idea could be exploring the backstory of the main character in 'A Taste of Rain'. Maybe they had a difficult childhood that influenced their actions in the story. Another could be a love triangle that develops between the main character and two other characters in this fanfiction world. It could add a lot of drama and tension.
Rain can signify a lot in Dark Gundam fanfiction. It could be a sign of cleansing, washing away the sins or the chaos that the Dark Gundam has brought. Or it could be the opposite, a sign of the spreading of the Dark Gundam's influence, like a dark omen that falls from the sky.
Well, perhaps they form a secret society within Hogwarts that promotes dark magic. They recruit students who feel left out or who have a thirst for more power. Dark Hermione could use her knowledge of spells and Dark Fleur her charm and allure to draw in followers. And then there could be a conflict when the other students and teachers start to suspect something and try to stop them.
One plot idea could be that Dark Naruto forms an alliance with Dark Kyuubi to take over the Hidden Leaf Village. They use their combined dark powers to manipulate the villagers and turn them against each other. Another idea is that they embark on a journey to find a powerful, ancient artifact that can enhance their dark abilities even further. Maybe they face off against the original Naruto and Kyuubi in a final battle of good versus evil.