Well, if Asta is kidnapped in a fanfiction, it could be a plot by some anti - magic group who see Asta's anti - magic as a threat. His sword might be taken away from him too. But Asta being Asta, he would try to fight back even without his sword. His determination would inspire any captives around him to also rise up against their captors. His friends would soon figure out who took him, probably through some detective work by Finral's spatial magic or Gauche's mirror magic.
If Asta were kidnapped in a fanfiction, it would be a huge turning point in the story. The Black Bulls would be in chaos at first, but then they would come together. Luck might be the first to recklessly rush into action to try and find Asta. Magna would be right behind him, ready to blast any bad guys. Meanwhile, Charmy would be using her food - related magic to keep everyone's energy up during the search. Asta's captors would likely underestimate his willpower and his friends' loyalty, which would ultimately lead to their downfall as the Black Bulls would stop at nothing to get Asta back.