Well, it could be a really interesting crossover. Spike could find himself in the world of the Powerpuff Girls. Maybe he'd be a bit confused at first, seeing all the different things. He might end up teaming up with the girls to fight some common enemy, like a new villain that combines the chaos of Equestria and Townsville.
In this fanfiction, Spike could bring his own unique abilities from MLP. His ability to breathe fire could come in handy when facing off against the villains in the Powerpuff Girls' world. And the Powerpuff Girls, with their superpowers, could teach Spike some new ways to use his powers more effectively. It would be a story full of action, friendship, and discovery.
He could fit in by being a source of different magic. Since he comes from Equestria, his magic could be different from what the Powerpuff Girls are used to. This could be used to counter some new threats in their universe. For example, if there's a villain who can nullify the girls' powers, Spike's magic could be the key to defeating them. He would also add a new dynamic to the group with his unique personality.
I'm not entirely sure as 'abuse' in fanfiction is a very concerning and often inappropriate topic. In general, My Little Pony (MLP) fanfiction should be positive and in line with the show's values. Spike is a character in MLP, and any form of abuse in fanfiction is not acceptable in a healthy fandom.
A possible plot could be centered around Spike's loyalty to Twilight. He discovers a secret that Twilight doesn't want anyone to know. Instead of thanking Spike for keeping it, Twilight blames him for finding out in the first place. This betrayal from Twilight, the one he admires the most, shatters Spike's world. He then has to decide whether to stay loyal or move on.
I'm not entirely sure as it could vary greatly. Fanfiction often takes the original 'Powerpuff Girls' concept and twists it in different ways. It might involve a darker take on the story where the characters meet an untimely death in a fictional scenario created by the fanfic writer.
One possible plot could be that Mojo Jojo discovers a new form of hypnosis and uses it on the Powerpuff Girls. He manages to make them believe they are his minions at first. But then, Blossom starts to break free from the hypnosis using her intelligence and realizes what is happening. She then tries to free her sisters Bubbles and Buttercup, and they all team up to defeat Mojo Jojo once again.
One possibility could be a story where Spike sacrifices himself to save his friends. Maybe there's a great danger in Equestria, like a powerful dark magic that only Spike can stop because of his unique dragon abilities. He might use his fire to counter the dark magic but in the process, exhausts all his energy and dies. His friends are left to mourn his loss but also remember him as a hero.
Perhaps the story could be about Spike turning into a girl and using this new form to her advantage. She could infiltrate an all - girl group or event that was previously off - limits to her as a male dragon. In this new form, she discovers things about herself that she never knew before, like a love for singing or dancing. Along the way, she also has to deal with the fact that she may or may not want to stay in this form permanently and what that would mean for her identity and her relationships.
In MLP Human Spike fanfiction, the stories can range widely. Some might focus on Spike's origin as a dragon and how that affects his new human life. He could have special powers or abilities that are either the same as in the pony world or altered in some way. There could be stories where he tries to hide his dragon past from his human friends, or he uses his knowledge of the pony world to solve problems in the human world. Fanfiction writers often take creative liberties to explore different aspects of Spike's character in a human form.
In a fanfiction where Spike is betrayed, it might involve Twilight Sparkle turning her back on him. Perhaps Twilight becomes so obsessed with her studies or a new magical discovery that she ignores Spike's needs and feelings. Spike, who has always been loyal to Twilight, would be deeply hurt by this perceived betrayal.
A likely kidnapper could be a rogue changeling. Changlings are known for their devious ways, and one might think that Spike has some sort of connection to the magic of love that they could use for their own purposes. So they kidnap him to study or exploit that connection.