He could form a friendly rivalry with Issei. Since Issei is all about power and his Boosted Gear, and Asta is super determined with his anti - magic sword, they could compete in terms of strength and growth. They might also learn from each other's combat styles.
Well, in some fanfictions, Asta's demon form might have a different appearance. Instead of the typical spiky look, it could be more streamlined and elegant, with a black and red color scheme that gives off an even more menacing vibe. Also, his demon form could possess the power to create illusions, which he uses to deceive his enemies. This would make him a more versatile fighter and add a new dimension to his battles.
His relationship with Sister Lily could be really interesting in fanfiction with his demon form. Since she has a kind and pure nature, she might be conflicted about Asta having a demon form. On one hand, she believes in Asta's good heart, but on the other hand, the idea of a demon form might go against her religious beliefs. This could lead to some really emotional and thought - provoking interactions between them, and Asta would have to prove to her time and again that his demon form doesn't change who he really is.
Asta would likely bring a new level of determination to the Crimson Lions. His never - give - up attitude could inspire the team members during tough battles.
Asta is still alive in the manga. The story keeps evolving with him at the center of the action. There have been many difficult situations, but he manages to overcome them without dying.