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.
Nero Wolfe in 'rex stout nero wolfe complete 47 novels' is a very unique detective. He is overweight and prefers to stay in his home most of the time, but his mind is sharp as a tack. He uses his deductive skills to solve various mysteries, often relying on his assistant Archie Goodwin to gather information for him. He has a passion for orchids and a refined taste in food, which are some of his distinct characteristics.
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.