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 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.
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.
In Danny Phantom, Danny himself is often a popular choice. His cool powers and heroic nature make him a great addition to any crossover. In My Little Pony, Twilight Sparkle is a common character in fanfiction. She's intelligent and powerful, so she could interact well with Danny.
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.
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.
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.
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.
First, you need to know the character well. Watch the show carefully to understand Danny Elliot's personality, his likes and dislikes. Then, come up with an idea for a story. It could be something as simple as a day in his life or a more complex plot like him getting involved in a mystery. Next, start writing. Don't worry too much about grammar at first, just get your ideas down. After that, go back and edit, make sure the story flows well and the dialogue sounds like something Danny would actually say.
Begin by choosing a genre for your fanfiction. Do you want it to be action - packed, a romantic story, or a mystery? Once you've decided that, think about the characters. Danny is the main character, but you can also focus on the secondary characters like Jazz or Valerie. Develop their personalities further in your story. For the plot, you can take inspiration from the show's episodes. Maybe there was an episode that left you with a lot of questions or ideas. Use that as a starting point. Also, make sure to give your story a unique twist, something that will make it stand out from other fanfictions.