One great 'Warhammer 40k Imperial Guard Fanfiction' is 'The Last Stand of the 13th Platoon'. It really shows the tenacity and sacrifice of the Imperial Guard. The way the characters are developed makes you feel like you're right there in the thick of the battle with them.
One popular one is 'Gaunt's Ghosts' series. It follows the Tanith First and Only regiment led by Colonel - Commissar Ibram Gaunt. The series is full of great military campaigns and character development.
One great Death Guard story could be about their unwavering loyalty in the face of extreme challenges. They are known for their tenacity and their ability to endure difficult situations. For example, in a battle against a powerful enemy force, the Death Guard held their ground despite being outnumbered. Their unique combat style and their use of diseased weaponry made them a force to be reckoned with.
Survival is another important theme. The Imperial Guard often face overwhelming odds. In novels such as 'The Last Chancers', the characters are constantly fighting for their survival in the most difficult situations. They have to use their wits, skills, and sometimes just pure luck to stay alive while serving the Imperium.
The main theme is often corruption. The Death Guard are chaos-corrupted, and this is shown through their actions and the spread of the plague. Another theme is loyalty, in a twisted way. They are loyal to their chaos gods and to Mortarion.
Mortarion is also a central character. His journey from a Primarch to a servant of Nurgle is a major part of the story. His decisions and actions shape the fate of the Death Guard as a whole.
The Horus Heresy Death Guard novels are great reads. One of the things that stands out is the detailed descriptions of the battles. You can almost smell the stench of the Death Guard's plagues as they spread across the battlefield. These novels also explore the relationships within the legion, like the bond between the marines and their leaders, which is really interesting.
One great Warhammer 40k Imperial Guard novel is 'Gaunt's Ghosts' series. It follows the adventures of Colonel - Commissar Ibram Gaunt and his Tanith First and Only regiment. The series is full of intense battles, great character development, and it really gives you a feel for the life and struggles of the Imperial Guard.
Another good place is the Warhammer 40k official forums. Fans often share their own fanfiction there. You might have to dig a bit, but you can find some real gems. Some of these stories might be in - progress, but it's still interesting to follow along.