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 She might become more withdrawn initially, but then find strength in wanting to carry on Asta's ideals.
One idea could be a story where Asta and Vanessa go on a special mission together. They face a powerful enemy and have to rely on each other's unique abilities. Asta's anti - magic sword combined with Vanessa's thread magic could create some really cool battle scenes.
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.
They could embark on an adventure to find a hidden magical artifact. This artifact could be said to have the power to enhance one's magic abilities even further. Asta would be eager to get it to become stronger and prove himself, while Vanessa would be more cautious but still go along to support her friend.
One common theme is power. Since Asta has a demon, many fanfics explore how he gains and controls new powers. Another theme is friendship. They often show how Asta's relationship with his friends is affected by his demon side. For example, some fanfics have his friends being initially scared but then coming to accept him.
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.