It could be a rival magic knight. Maybe someone from a different squad who is jealous of Asta's rapid rise in the ranks and the attention he gets. They might think that by kidnapping him, they can make themselves look better or get rid of the competition.
It could be that the kidnappers have a personal vendetta against Asta. They might be from a family or group that was shamed by Asta's actions in the past. In this situation, they might not be interested in his anti - magic directly. Instead, they just want to get revenge on him. Asta, being the positive person he is, would probably try to talk some sense into them. But if that doesn't work, he'll have to rely on his physical strength and whatever anti - magic powers he can muster up to defend himself until help arrives.
The relationship could change in terms of trust. Noelle, being sick and vulnerable, has to trust Asta to take care of her. Asta, in turn, has to be responsible and trustworthy. This mutual trust - building experience can strengthen their bond. Also, the way they interact during this time might set a new pattern for their future interactions. For example, if Asta is gentle and patient with Noelle while she's sick, she may expect the same level of kindness from him in the future, and he will be more likely to continue to show it.
In the fanfic, since Noelle is sick, it could be a chance for their relationship to develop further. Asta, being the energetic and kind - hearted guy he is, would probably do some silly things to cheer her up. And Noelle, in her weakened state, might let her guard down more around Asta, leading to some really sweet and tender moments between them. For example, she could share some of her insecurities with him while he holds her hand reassuringly.
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.
One challenge would be managing the different personalities. The Black Bulls have a wide range of characters, from the serious Noelle to the wild and unpredictable Finral. Asta will have to find a way to keep everyone in line and motivated.
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.