Given the very nature of fan fiction, it is safe to assume any character can die in any story.
Avatar: The Last Airbender Revised firmly established itself as possessing Anyone Can Die from the beginning, with the passing of Sai's friend Xin Fu, Sakodi's friend Enri, and numerous Kyoshi warriors, among others, in the first book alone. By the end, Ain, Hakoda, Teo, Azul and Aizo are dead, and Zotu is Faking the Dead.
The author of Bad Future Crusaders, although promising she won't just kill characters left and right for the heck of it, has made a point of claiming that no character is safe since it makes for a far more interesting read. Seemingly to hammer this point home, this was said right after Twist who was built up as a major character, was killed very unceremoniously by Merrilay.
The Blood Bond, Blood Omen Series of fanfics for Kim Possible kills off familiar characters with reckless abandon. Among the casualties are Dr. Director, Monkey Fist, Gemini, Sensei, Frederick, Hirotaka and Adrena Lynn's face.
Crossover series Both Syllables has so far killed off three fairly major characters. Two of these were solely because the author is irritated by the amount of Disney Deaths in the source material.
The fanfic series A Dance of Shadow and Light by Ocadioan plays this trope to a T after the third story, where the author gives no thought to killing off characters(both minor and major) left and right, like the time when a sister-in-law and mother to a main character's niece dies off screen and for no apparent reason or purpose, as opposed to the first story where only a reluctant Dragon dies near the end, and the second story where only the male main character's established love interest was killed by his own hands, though that was more a case of I did what I had to do.
Dangerverse. Similar to the original series, though by the end of book 4 it nearly rivals the whole series. Quirrell, Lockhart, Siss, Andromeda, Cedric, Marcus, Umbridge, Frank Longbottom, Penelope Clearwater, a little girl not even in Hogwarts yet, Hagrid, and the last book isn't done yet.
Downfall takes a sharp turn from the original source, where deaths are unheard of; or at least, don't stick. Though several characters start out alive and kicking, when in canon they were dead and buried long before the series started, the second major engagement in Unohana's war sees Rukia dead.
In Dumbledore's Army and the Year of Darkness, in the Battle of Hogwarts, the brutal deaths of Dennis, Michael, Cho, Parvati and too many others come as shocks. Continued with alarming enthusiasm in the sequels. Basically, anyone who did not get an explicit name check as being alive and well in Rowling's epilogue can be presumed to be dead by the time the trilogy has run its course. And even that is no guarantee, because "Epilogue? What Epilogue?"
In the pony fanfic Envy and Arrogance, this is expressed in three stages. First, most of the population of Equestria dies off-screen, including several minor characters. Then, the stakes are taken higher with a Tear Jerker death of Apple Bloom. More deaths follow. Finally, we learn that the original Elements of Harmony managed to save the world despite losing several members (either to death or mental breakdown) - that means even the mane six's Plot Armor can no longer be depended upon.
In Future Tense, it starts with three characters dying within the first few paragraphs and only goes downhill from there.
With the Gensokyo 20XX series being a series of primarily Dark fics and Death Fics, it comes with this trope and even immortals and children are no exception, seeing as Eirin, being one of the aforementioned, was Driven to Suicide and succeeded and the fact that the series has a rather high body count.
Happy Families Are All Alike features the Third Hokage dying in the very first chapter. Later, we find out that Jiraiya and Tsunade were killed off by Orochimaru, who has also successfully possessed Sasuke.
The Ace Attorney fic Law Plus Chaos has many characters killed off, most of them in VERY brutal ways, such as one person being cooked into pies.
Played with in Series Five of The Lion King Adventures. Tojo and Tama die in grisly ways, but – unlike some others who are brought back – their deaths remain permanent.
The Fanfic Pedestal. Human or Pokemon, Original or Canon Character. Anyone could be next.
The Sun Soul, a Pokémon fic, fits this trope in every way possible. The author spares no pains in developing a character, making the audience identify with them, then ripping the character away in a brutal, often jarring, manner.
The Command & Conquer fan-novelization Tiberium Wars makes it a point to often, suddenly, and repeatedly kill off well-developed characters. Some characters get less than a scene to their names before they get slaughtered, and you've really got no idea if a character is going to survive until they bite it.
Most of Hans Von Hozel's fics end on a melancholy note. For example: He kills all of the Ninja Turtles and Splinter in his story Ninja Turtles and in his Doctor Who fic, the Doctor gets eaten by raptors.
Zombies, a videogame crossover where a Zombie Apocalypse is corrupting the world of video games. Anyone who gets bitten or infected will become a zombie, will gain superhuman strength, and a relentless urge to kill and devour the living. And the best place to do this is in the idyllic Mushroom Kingdom where even turtles can pose a significant threat. Almost anyone who might seem important is on the author's hitlist.