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.
A potential plot could be Asta's slow descent into darkness. He starts off as a bit more cynical after facing continuous rejections. Then, he meets a group of outcasts who have also been wronged by the system. They convince him that the only way to make things right is to take power by force. Asta begins to use his anti - magic to steal magic from other mages, believing it's a way to level the playing field. As he does this, he becomes more and more feared and is seen as a threat to the kingdom.