Horus is often very popular in fanfiction. His fall from grace is a major event in the Warhammer 40k lore, and many fanfiction writers like to explore the reasons behind it more deeply. They might create alternate timelines where he doesn't fall or explore his relationships with other Primarchs in more detail.
Some fanfictions might center around the battles between different Primarchs. For instance, the confrontation between Horus and Sanguinius. In these stories, the authors often delve into the tactical aspects of their warfare as well as the emotional turmoil of the Primarchs. Additionally, there are fanfictions that show the Primarchs interacting with normal Space Marines in unique ways, perhaps mentoring them or learning from them in some cases.
Another popular interpretation is that the lost primarchs were secretly working against the Imperium all along. This could involve them allying with chaos or other enemies of the Imperium, and their disappearance was part of a long - term plan to undermine the Imperium from within. Fanfictions based on this idea often involve complex plots and political intrigue within the Warhammer 40k setting.
Another concept is the potential power they might have had. Given that the known primarchs were incredibly powerful, the lost primarchs could have had unique abilities that would have changed the course of the Warhammer 40k universe. For example, they might have had a different approach to dealing with the xenos threats, which could have led to a very different state of the galaxy.
Ava is also quite popular. She is usually written as a brilliant strategist. Her fanfiction stories often involve outsmarting her enemies through complex and innovative battle plans. She is seen as a symbol of female intellect in the world of the primarchs.
There's a fan - created female primarch called Selene. The writer has given her a complex personality. She is both a fierce warrior and a diplomat, which makes her character very appealing in the fanfiction world.
One interesting idea could be the unique leadership styles of female primarchs. They might bring a more collaborative and empathetic approach compared to their male counterparts in the fanfiction. For example, they could focus on building stronger communities within their legions rather than just military might.
Since it's a fanfic about lost primarchs, the main characters are likely to be the lost primarchs themselves. However, it could also include other characters who are affected by their loss or the mystery surrounding them, like the remaining primarchs or some of the key military leaders in the story's universe.
Probably 'Horus Rising' by Dan Abnett. It's a very popular one as it sets the stage for the Horus Heresy, which is a huge part of the Warhammer 40,000 lore. It gives a great introduction to Horus as a Primarch before his fall.
Perhaps a key idea in this fanfic is the mystery surrounding the lost primarchs. Their very existence being a secret could be used to create a sense of intrigue. The fanfic might explore how rumors and half - truths about them spread among different factions, and how these rumors influenced the actions and beliefs of various characters.