One plot idea could be Yami going on a solo mission to a mysterious and dangerous land. He has to face powerful new enemies and discover secrets about his own past. Another could be Yami training a young and inexperienced magic knight, sharing his unique combat and magic knowledge.
Yami remains alive in the manga. The author chose to keep Yami as an ongoing character to drive the story forward and interact with other characters in various scenarios.
As far as I know, Yami doesn't die in the manga. The story might have twists and turns, but that particular event hasn't happened yet. Maybe there are rumors or speculations, but based on the official storyline, no.
To write a believable characterisation of Yami, focus on his rough exterior but also his loyalty to his squad. Show his love for food, especially Japanese food. Also, don't forget his unique magic style and how he uses it in combat.
In the fanfiction, Yami suddenly falls ill. His friends notice his pale face and lack of energy. They quickly bring him to a healer. The healer diagnoses him with a strange fever. As Yami lies in bed, weak and delirious, his friends take turns to look after him, bringing him warm soup and medicine, hoping for his speedy recovery.
One common theme is adventure. Given their abilities and the worlds they come from, many fanfictions send them on exciting quests. For example, they might be on a journey to find a lost magical artifact.