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.
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.
Well, it could be that Spike feels unappreciated in Ponyville. Maybe he's tired of always being seen as just Twilight's assistant. So he runs away to find a place where he can prove himself and be his own dragon.
In the fanfic, it might also be about Spike having some sort of problem, like he accidentally gets into trouble with a magical artifact. Big Mac, being the reliable and steady pony he is, steps in to help Spike figure out how to fix the situation. This could involve them going to visit Twilight Sparkle for advice and along the way, they face various challenges that test their bond and also show their individual personalities.
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.
Well, it could be about Spike's adventures as a pony. Maybe he discovers new magical abilities that are unique to ponies. He might make new pony friends and go on quests with them in Equestria.
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.
One possible plot could be that Spike discovers some ancient magic in the dragon lands. This magic starts to manifest in a strange way, causing him to grow wings. At first, he's scared and confused, not knowing how to use them. But with the help of his pony friends like Twilight, he starts to learn how to fly. Along the way, he might face some challenges from other dragons who are jealous of his newfound ability.
Well, it could be that Twilight discovers some sort of dark secret about the Mane 6 and decides to betray them for what she believes is the greater good. Maybe she's been influenced by some external force.
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.
He could meet other dragons. Since he is a dragon himself, it would be natural for him to encounter some of his own kind during his runaway journey. They might teach him about dragon culture and traditions that he doesn't know much about.