In certain fanfictions, a jealous rival of Amy Rose could be the kidnapper. This could be another female character who has feelings for Sonic and sees Amy Rose as a threat. She might think that by getting rid of Amy Rose, she can have Sonic all to herself. Or it could be a character from Amy Rose's past who has some unresolved issues with her and decides to kidnap her to get revenge or make her pay for something that happened a long time ago.
One common plot could be that Amy Rose is kidnapped by a villain who wants to use her as bait to lure Sonic. The kidnapper might think that Sonic would come to save her, and they could set up a trap for him. Another plot could be that the kidnapper has a personal vendetta against Amy Rose herself, perhaps because she got in the way of their evil plans before. There could also be a plot where Amy Rose is kidnapped by mistake, and the kidnapper realizes their error but still decides to hold her captive for some other reason, like ransom or information.
Even one of her friends like Tails could be the betrayer in a really unexpected plot twist. Maybe Tails is forced to betray her by some external threat. For example, if an enemy captures Tails and threatens to harm him or Sonic if he doesn't give away some important information about Amy Rose or lead her into a trap.
A 'amy rose dies fanfic' could be a very sad and dark take on the character. It might explore themes like loss, grief among the other characters who knew Amy Rose. Maybe it's a way for the writer to show how her death could impact the whole story's universe.
I highly doubt that 'amy rose poop fanfic' is a popular type of fanfic. Most people are more interested in fanfics that focus on positive aspects of the characters like their relationships, adventures, or character development rather than something as unappealing as poop.
To write an engaging Amy Rose birthday fanfic, think about the setting. It could be in the Sonic universe, like in Station Square or Angel Island. Describe the place in detail. For the plot, you could have a mystery where Amy's birthday presents go missing. Her friends suspect a new villain, but in the end, it turns out to be a misunderstanding. This creates drama and keeps the readers interested.
One idea could be a story where all of Amy Rose's friends plan a surprise party for her. They decorate the entire Green Hill Zone with her favorite flowers and prepare a huge cake in the shape of her Piko Piko Hammer.
A common theme is love. Since Amy Rose has a strong love for Sonic in the Sonic universe, many fanfics explore her love story, whether it's a successful romance or her dealing with unrequited love. Another theme is friendship. Her relationships with other characters like Tails or Knuckles are often explored.
Yes, there are many appropriate fanfics related to Amy Rose. Some might focus on her adventures, her relationships in a friendly or heroic context, like her interactions with Sonic and their friends in different storylines that involve saving the world or having fun adventures together.