Well, 'Guilliman: Lord of Ultramar' is also a very good one. It gives a deep look into his character, his upbringing in Ultramar, his values that were instilled in him from a young age, and how those shape his actions and decisions throughout his life.
A common theme is leadership. Guilliman is a great leader, so many fanfics explore how he leads in different situations, like during battles or when dealing with political unrest in the Imperium.
One great piece of Roboute Guilliman fanfiction is 'The Primarch's Return' which delves into his post-resurrection experiences and how he readapts to the Imperium. It really captures his essence as a leader and strategist.
I don't have the exact answer off the top of my head. It could potentially be in one of the more recent series or standalone novels, but you'd have to check the detailed plot summaries or consult with other avid fans of the Black Library.
Historical anime caricatures often combine elements of history with creative and exaggerated visuals. They might feature unique art styles and tell stories in a way that catches the viewers' attention.