Since lemon fanfics are inappropriate, we should focus on their general characteristics from the official story. Asta is highly determined and energetic, always striving hard despite his lack of magic initially. Yuno, on the other hand, is more calm and has a natural talent for magic. They both have a strong bond as rivals and friends.
Noelle could be a great addition. Her water magic is quite powerful and she has a complex relationship with both Asta and Yuno. She could be a source of both support and conflict in the fanfic. Another good character is Luck. His lightning magic and his rather unpredictable nature can add a lot of action and excitement to the story.
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.
Their relationship can be developed through shared challenges. For example, when they are faced with a difficult mission, they have to trust each other's skills. Asta can rely on Charlotte's more refined magical abilities and Charlotte can start to trust in Asta's brute - force - like anti - magic power. This mutual trust can be the foundation for a deeper relationship.
Asta in an elf fanfic might be the protagonist. He could be on a quest, perhaps to save the elves from some great danger or to discover a hidden secret within the elven realm. He might form alliances with different elven clans and face many challenges along the way, like fighting against dark magic or evil creatures that threaten the elven way of life.