Eren could be an interesting choice. Since he has such a complex character arc already, making him the tenth titan could add another layer to his story. He could struggle with the new power and how it changes his view of the world and his mission.
I'm sorry I don't know precisely who penned the first Warhammer novel. However, over the years, there have been numerous authors involved in the Warhammer series. It would require looking into the history of the Warhammer novels' publication to find out the exact author of the very first one.
In the 'Horus Heresy' series, Horus Lupercal is a central character. He was once a great and noble Primarch who ultimately turned to Chaos. Another important character is Roboute Guilliman, the Primarch of the Ultramarines. He is known for his strategic genius and his efforts to preserve the Imperium.
Armin could also be a main character in such a fanfic. He's known for his intelligence. As a permanent titan, he could use his smarts in unique ways. He might try to figure out the secrets of the titan form from the inside and communicate with the other characters in unexpected ways. For example, he could leave clues or messages for his friends using his titan abilities.
Eren could be an interesting main character in a friendly - titan fanfiction. He has a complex relationship with the titans already. Imagine if he discovers a friendly titan that changes his view of all titans. He could try to understand it better and maybe even learn to control other titans through the knowledge he gains from this friendly one.
Titanfall 2 was a sci-fi first-person shooter game produced by Respawn Entertainment and published by Electronic Arts. This new game introduced a new single-player campaign, and players would explore the bond between Iron Warder and Titan. In addition, players could also experience more innovative and exciting multiplayer modes, including six new Titans, new Iron Rider skills, expanded customizations, new maps and game modes, and more. There was almost no difference between battle mode and battle mode in the game, but the way the story progressed in battle mode might make the player ignore the conversation and reduce the sense of immersion. Titanfall 2 received positive reviews after its release, and sales reached 10 million players.
I'm not entirely sure who 'Percy' is as the Titan in this specific dark fanfiction without more context. It could be a unique character created by the author with their own set of powers and characteristics within the fictional world of that fanfiction.
Since I don't know the specific 'titan novel', I can't say for sure who the main characters are. They could be heroes fighting against Titans if it's a myth - based story, or astronauts exploring Titan if it's a science - fiction one.