Yuno would be completely shattered. He has always seen Asta as his rival and friend. He would probably blame himself for not being strong enough to protect Asta. His grief might turn into a cold determination to change the world in Asta's memory. Noelle would be heartbroken. She has deep feelings for Asta, and his death would be a huge blow. She might become more withdrawn initially, but then find strength in wanting to carry on Asta's ideals.
Asta's age in the Black Clover story arc changes over time. At the beginning, he's around 15 years old, and as the story goes on, he gets older. The exact age depends on the particular events and chapters.
One great 'Black Clover Asta x Rebecca' fanfic is 'Asta's Promise to Rebecca'. It beautifully explores their relationship in a world full of magic and adventure. The author really captures Asta's determination and how Rebecca sees him in a new light.
Another theme is growth. Asta is constantly growing in his power and abilities, and Rebecca also grows as a person. Their relationship is a part of that growth. In some fanfics, they help each other overcome personal limitations, which is really inspiring.
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.
Perhaps a powerful dark mage who has been observing Asta. When he loses control, this mage could try to manipulate Asta's out - of - control anti - magic to his own advantage. He might want to use it to destroy the Magic Knights or gain power over the entire kingdom. This mage could be someone who has a vendetta against the Clover Kingdom and sees Asta as a useful tool in his evil plan.