One idea could be a crossover where Danny Phantom's powers are needed in Equestria to fight off some sort of dark magic. Maybe the ponies mistake his ghostly form for a new type of magical creature at first.
Another interesting concept is that Danny, for some reason, gets transported to the world of My Little Pony and has to learn the ways of the ponies while also trying to find a way back home. Along the way, he could use his powers to help the ponies in their daily lives, like using his intangibility to reach things that are too high or his invisibility to play pranks on some of the more mischievous ponies.
One idea could be that Danny Phantom accidentally gets sucked into the My Little Pony world. He could use his ghost powers to help the ponies solve a mystery that's been haunting their land, like strange disappearances that are actually caused by a mischievous ghost pony. Danny would have to adapt to the colorful and friendly world of ponies while also trying to find a way back home.
A more complex concept could be a fusion of the two universes where the elements of the My Little Pony world, such as friendship and magic, interact with the ghostly elements of Danny Phantom's world. For example, the magic in the pony world could be used to enhance Danny's powers or to help him control his ghost side better. And Danny could introduce the ponies to new ways of thinking about things that are different from their normal, more positive and friendly world view.
One great 'My Little Pony and Danny Phantom fanfiction' is 'Phantom in Equestria'. It has an interesting plot where Danny somehow gets transported to the world of My Little Pony. Another one could be 'The Ghostly Pony' which combines the unique powers of Danny with the magical world of ponies in a really creative way.
One common theme is the exploration of different powers. Danny has his ghost powers and the ponies have their magic, so fanfictions often explore how these powers interact. Another theme is friendship. Just like in their original shows, the stories usually focus on how characters from different worlds can become friends and support each other.
One idea could be that Danny gets an illness that affects his ghost powers. For example, he might start losing control over his intangibility or invisibility. This could lead to some really comical or dangerous situations. Another idea is that his friends try to find a cure for his strange ghost - related illness while also keeping it a secret from the town.
I'm not sure of specific titles off the top of my head, but you can try searching on fanfiction - specific websites. Look for ones with high ratings and lots of reviews. Usually, those are the better - quality ones.
One idea could be exploring Danny's relationships in a different way. For example, how his friendship with Sam and Tucker might develop if they faced a new supernatural threat. Another could be a story where Danny discovers a new power and has to learn to control it while dealing with school and his ghost - fighting duties. And perhaps a fanfiction where Danny travels to an alternate dimension within the Danny Phantom universe.
Danny and Twilight Sparkle would make an interesting pair. Danny with his ghost - fighting skills and Twilight with her powerful magic. They could exchange knowledge about their different worlds and powers. Danny could learn about magic from Twilight, and Twilight could be fascinated by Danny's ghost powers.
A great fanfiction idea could be a time - travel story within the Danny Phantom Phantom Planet universe. Danny somehow gets sent back in time to before he became a half - ghost. He has to figure out how to get back to his own time while also dealing with the knowledge he has of the future. This could lead to some really interesting interactions with his past self, his family, and friends. And he might accidentally change things in the past that could have a huge impact on the future of the Phantom Planet.
One idea could be a story where Sonic accidentally runs into the My Little Pony world. He could team up with Twilight Sparkle to stop an evil force that's threatening both their worlds. Maybe a chaos - causing villain has found a way to cross between the two realms.