Silver is often depicted as very determined in fanfictions. His strong will to achieve his goals can be an inspiring quality. He also has a kind heart, which makes him a likable character in relation to the reader. For example, he might be shown as being protective of the reader.
The 'Dark Silver the Hedgehog story' could be a really interesting and complex one. If we assume it's a new take on the Silver the Hedgehog character, perhaps he discovers some dark secrets about his past or his world that lead him down a different path. Maybe he has a conflict with his friends or allies who don't understand his new 'dark' attitude. He could be on a journey to either embrace this new side of himself or fight against it to return to his original self. It might also involve a new set of enemies that are attracted to his 'dark' energy, creating a whole new set of battles and storylines.
Destiny is also a big theme. Silver is often depicted as trying to fulfill his destiny, whether it's saving his world or something greater. Additionally, the concept of hope is common. Despite the difficulties he faces, he usually holds on to hope, which is a central part of many fanfictions about him.
One common theme is time - travel, as Silver has time - traveling abilities in the games. Fanfictions often explore different ways he can use this power. Another theme is friendship, like how he forms bonds with other characters from the Sonic universe. For example, his relationship with Sonic and how they learn to work together.
Since I'm not sure of the exact story, I'd say one key element could be Silver's transformation into a 'dark' version. Another might be his new powers or abilities that come with this change. And of course, his new relationships with other characters, whether they are enemies or those who try to bring him back from the dark side.
The character of Silver himself. He has these cool psychic powers. Fanfics can explore how he uses them in different situations that the official games might not have shown.
Perhaps a new, unknown villain emerges and kidnaps Silver to lure Sonic or the other heroes. The kidnapper could be after Silver's psychic powers, thinking they can be used for some evil plan. The plot could then revolve around Sonic and his friends trying to rescue Silver and stop the villain's scheme. It might also involve Silver using his own wits and powers from within his captivity to either escape on his own or send a signal for help.
Silver the Hedgehog is often portrayed as a complex character. In these sad stories, perhaps he's dealing with time - related issues. He might be sad because he can't change certain events in the past or future, no matter how hard he tries. His time - traveling abilities could be a curse rather than a blessing in these tales.
One common theme is empowerment. The female Silver is often depicted as a powerful character who can take on challenges. For example, in many fanfics, she uses her psychokinetic powers to fight against evil forces.