There is also Cawl. He is an interesting character, known for his technological prowess and his role in various projects within the Imperium. And then there are the characters on the side of chaos who oppose Guilliman and the Imperium, like various daemonic entities.
Power and corruption are key themes. The novels often explore how power within the Imperium can lead to moral decay. Another theme is the constant struggle against chaos, which is always threatening to overtake the Imperium.
One of the key characters is Guilliman. He is a significant figure in the Imperium, with his leadership skills and strategic acumen being crucial. Another is Mortarion, who represents the forces of Chaos and is a formidable adversary.
The theme of loyalty is also prominent. Characters have to decide where their loyalties lie, whether it's to their superiors in the Imperium, their comrades, or to their own ideals. There is also the overarching theme of survival in a brutal and unforgiving universe where death lurks around every corner.
The novels also focus on the internal struggles within the Imperium. There are power plays, different factions vying for control, and the challenges of maintaining order in a universe filled with chaos. For example, the different Space Marine Chapters may have different views on how to deal with the emerging threats, which causes some tension.
The antagonists from the forces of Chaos are also key in a way. Characters like the Chaos Lords or daemonic entities create the conflict. Their actions and plots drive the story forward as the Imperium has to counter their threats. Their motives of spreading chaos and destruction set the stage for the intense battles and struggles in the novels.
The story of Warhammer 40k Dark Imperium is centered around the Imperium of Man at a very crucial time. Guilliman, a significant figure, has resurfaced. His return shakes things up as the Imperium has been in a rather dire state. There are the forces of Chaos constantly attacking from all sides. Guilliman tries to bring order back to the Imperium, which has been fragmented over time. The plot also shows how the Imperium's military forces, especially the Space Marines, adapt to the new leadership and the challenges that come with it. It's a story full of battles, political intrigue within the Imperium, and the never - ending fight against Chaos.
One great 'Imperium of Man Fanfiction' is 'The Emperor's Gift'. It offers a unique perspective on the Imperium, exploring the relationship between different Space Marine Chapters. Another good one could be 'In the Service of the Imperium' which delves deep into the lives of the Imperial Guardsmen and their daily struggles. And 'A Glimpse of the Imperium' is also worth checking out as it presents a broader view of the Imperium's politics and power dynamics.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of 'Imperium of Man Fanfiction' with various themes and styles. Another option is FanFiction.net, which also has a decent number of such stories. Just search for 'Imperium of Man' in the search bar and you'll find a bunch of them.