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.
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.
Well, 'mlp' stands for My Little Pony. Spike is a character from that franchise. A 'fanfic' is a fictional work written by a fan. However, the 'diaper' part makes it a bit strange. It could potentially be a very unique and niche fan - created story where Spike has some sort of adventure or situation that involves diapers. Maybe it's a comical take on the character, or perhaps it's a more serious exploration in a very different kind of story setting, but again, this is all speculation without actually reading the fanfic.
In a 'evil spike mlp fanfic', it might be that Spike has been influenced by some external dark force. This could lead to him betraying his friends and working against the values of friendship that are so central to My Little Pony. The fanfic could delve into his motives, like if he feels unappreciated or overlooked, and then show how his evil deeds play out and whether there's any chance for redemption in the end.
With the dragons, Spike's status could be different. Dragons that used to look down on him might now be either more respectful or more hostile out of envy. Among the ponies, Spike would become even more of a novelty. Some ponies might ask him for rides or to help them reach high places. His relationship with Rarity could also be affected. She might be inspired to create new outfits for him to match his new look with wings.
Well, Pinkie Pie is also a likely main character. She would be the one to throw a big 'Welcome to Pony - hood' party for Spike. She could also introduce him to all the fun and wacky things about being a pony. Rarity could be involved as well. She might take it upon herself to give Spike a makeover in his new pony form, making him look dashing and stylish among the ponies.
One idea could be that Spike and Sweetie Belle team up to put on a play for the Ponyville residents. Spike can use his fire-breathing ability to create special effects, while Sweetie Belle showcases her singing talents.
To develop their relationship, you can write about how they initially have some misunderstandings. Spike might think Sweetie Belle is too flighty, while Sweetie Belle may think Spike is too uptight. But as they spend more time together, they realize they have a lot in common, like their love for their respective sisters and their dreams of making a mark in Ponyville. They start to support each other in achieving their goals and become great friends.
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.