One popular Danny Phantom injured fanfiction might be 'The Battle's Aftermath'. It focuses on Danny's recovery after a particularly tough fight with a powerful ghost. The author does a great job of showing his physical and emotional pain, and how his friends and family support him through it.
There's also 'Injured in the Shadows'. This fanfic delves deep into Danny's internal struggle when he gets seriously injured while protecting the town secretly. It's interesting as it shows how he tries to hide his injuries from his loved ones, fearing they'll stop him from ghost - fighting.
One possible scenario could be Danny getting injured during a fight with a powerful ghost. Maybe he underestimated the ghost's new power and got hit by a strong energy blast that left him with a serious injury.
One great Danny Phantom Undergrowth Sam fanfiction is 'The Green Bond'. It delves into Sam's connection with the Undergrowth and how it affects her relationship with Danny. Another interesting one could be 'Sam's Undergrowth Adventure' which shows Sam exploring the mysteries of the Undergrowth realm. There's also 'Danny, Sam and the Undergrowth Threat' where they team up to face a danger emerging from the Undergrowth.
One great fanfiction could be where Sam and Dean are on a hunt and accidentally summon Danny Phantom. They initially think he's some sort of demon but soon realize he's not. Danny helps them with their supernatural problems using his ghostly powers. Another could be a more comical one where Danny is summoned and he's just super confused about being in the 'Supernatural' universe and the chaos that ensues is hilarious. There's also the idea of a darker fanfiction where the summoning goes wrong and Danny becomes corrupted and Sam and Dean have to figure out how to fix him.
One popular Danny Phantom fanfiction is 'Phantom's Shadow'. It has an interesting plot where Danny faces a new, more powerful ghostly adversary. Another is 'Danny's New World' which explores a different dimension where Danny has to adapt to new rules and powers.
Fanfiction.net could be another place. Just type in 'Mother Danny Danny Phantom' in the search box. However, the quality can vary greatly. Look for ones with high ratings and a good number of reviews. Some Tumblr blogs also reblog Danny Phantom fanfictions, so it might be worth checking out some Danny Phantom - related Tumblr tags.
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.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. These platforms have a wide variety of fanfictions. Just search for 'Danny Phantom Arrancar' and you'll get a list of relevant stories.
One great Danny Phantom and Danielle Phantom fanfiction is 'The Twin's Adventure'. It explores their relationship in a new ghost - fighting scenario. Another is 'Danielle's Discovery' which delves into Danielle finding out more about her powers and her connection to Danny.
Maybe it's one of his old enemies who has found a new way to attack him. For instance, Vlad could come up with a more devious plan that manages to catch Danny off - guard and injure him. Vlad has always been a formidable foe, and he might use Danny's overconfidence or a moment of weakness against him.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often dedicated sections for both 'Secret Saturdays' and 'Danny Phantom' fanfics. Just search for the relevant tags, and you'll be presented with a plethora of options.