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.
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.
Maybe it has the ability to absorb chakra. So when Naruto is in a fight against an opponent with a large amount of chakra, he can use the sword to drain some of that chakra, weakening the enemy while strengthening himself. The sword could then store that chakra for later use, like to boost Naruto's own attacks.
It could be a sword that can cut through any substance, like the Susanoo easily. Just imagine Naruto swinging it and slicing through powerful jutsu defenses as if they were paper.
Asta is still alive in the manga. The story keeps evolving with him at the center of the action. There have been many difficult situations, but he manages to overcome them without dying.