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.
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.
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.
Power is a very common theme. Since Yami Marik is a powerful character, many fanfictions explore how he uses his power, gains more power, or loses it. Another common theme is revenge. Given his backstory, it's often a part of the story. For example, he might seek revenge on those who wronged him in the past.
The so - called 'naruto yami lemon fanfiction' might have some characteristics that deviate from the mainstream Naruto fanfictions. 'Yami' could mean that the stories have a more gloomy or sinister tone, perhaps with characters facing more extreme and cruel situations. However, as for the 'lemon' part, it is not something that should be part of a proper fanfiction. Naruto is a story about friendship, growth, and ninja adventures, and these inappropriate elements don't belong in it.