One great one to start with could be 'The Ghostly Guardian'. It really delves into Danny's growth as a hero. Another good option is 'Phantom's New Beginnings' which gives an interesting take on his relationships. And 'Danny's Secret Allies' is also quite engaging as it introduces some new and unique characters.
One interesting Danny and Dani fanfiction could be about them teaming up to fight a new and powerful ghost. They use their combined powers and unique abilities to outwit the enemy. For example, Dani might use her speed while Danny uses his more powerful ghost ray.
You can start by checking out fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Danny Phantom' and then sort by the ratings and reviews. The ones with high ratings and positive reviews are likely to be high - quality.
The 'Danny Phantom novel' probably focuses on Danny's journey. Danny is a half - ghost, half - human. So it may show how he copes with his dual nature. It may have stories of him using his powers like invisibility and intangibility to solve problems. It might also involve his school life and how he hides his powers from his classmates while still dealing with the everyday challenges that come his way in the ghost - fighting business.
In a crossover, characters from the other franchise might betray Danny. For example, if it's a crossover with a show about vampires, a vampire character who seems friendly at first could betray Danny for his ghost powers. They might want to use his powers for their own dark purposes. Another likely candidate could be a character who is jealous of Danny's abilities. Maybe in a crossover with a superhero world, a sidekick who feels overshadowed by Danny's powers and popularity might be the one to betray him.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often a lot of Danny Phantom stories there, and you can specifically search for ones involving Danny and Dani. Fanfiction.net is also a good place to check. Just use the search function and type in 'Danny Phantom Danny and Dani' and you should get some results.
It could be Sam and Tucker. Since Danny has become a villain, his former friends would likely try to stop him. They would be the main antagonists from Danny's point of view now. They would use their knowledge of Danny and their own skills to try and bring him back to the good side.
I think it could be Superman. Superman is all about truth and justice and he would likely see the good in Danny. Also, they both have super strength in a way. Danny has his ghostly strength and Superman has his Kryptonian strength. They could bond over protecting the innocent.
A Danny Phantom hurt fanfiction could involve Danny getting hurt while exploring a new, dangerous part of the Ghost Zone. He might encounter some sort of toxic energy or a trap set by an evil ghost. His pain and struggle to get back to safety could be the main focus, and how he overcomes his injuries could show his strength and determination.