A Solitaire from the Harlequins could be a great addition. They are mysterious and highly skilled warriors. In a crossover, they could add an element of mystery and their unique combat skills could make for some exciting action scenes, especially when pitted against the Dark Eldar.
A Rogue Trader from the Imperium would also be a good character to include. They are known for their adventurous and often unscrupulous nature. In a crossover with the Dark Eldar, they could be either trying to make a deal with the Dark Eldar for some rare and valuable items, or they could be trying to stop the Dark Eldar from raiding Imperial worlds. Their lack of strict adherence to Imperial dogma gives them more flexibility in their interactions with the Dark Eldar.
The Dark Eldar Archon is an obvious choice. He is a powerful and cunning leader in the Dark Eldar society, full of ambition and a thirst for power. His complex motives and devious nature can drive a lot of the plot.
One great 'Warhammer Dark Eldar Fanfiction' is 'The Darkest Soul'. It delves deep into the psyche of a Dark Eldar character, showing their struggle between their cruel nature and a glimmer of something more. The author does an amazing job of painting the dark and twisted world of the Dark Eldar.
One idea could be a story about a Dark Eldar wych who is trying to rise in the ranks within her cult. She could face challenges from rival wychs and have to prove her worth in the gladiatorial arenas of Commorragh.
The Archon is often a popular character. His power and devious nature make for great stories. He's usually at the center of power plays and schemes in Commorragh. Another popular one is the Succubus. With her combat skills and allure, she can be a central figure in stories about wych cults and battles.
Perhaps a Dark Eldar wych could form an unlikely alliance with a Space Marine. Maybe the wych has a personal vendetta against a chaos warband that is also targeting the Space Marine's chapter, and they decide to work together to take down the common enemy. This would involve a lot of tense moments as they try to trust each other despite their very different cultures and motives.
Well, for a simple start, you could write about a Dark Eldar Kabalite warrior's journey. He might be sent on a mission to capture a valuable psyker from a human world. It would be full of action as he battles the Imperium's forces protecting the psyker.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of Warhammer fanfictions, including many about the Dark Eldar. Just use the search function and type in 'Warhammer Dark Eldar' and you'll get a bunch of results.
Grey Seer Thanquol is a great choice. His devious nature and ambition can drive a lot of the plot in a crossover. He could be the mastermind behind the Skaven's actions in the new world they cross into.
In many of the novels, Asdrubael Vect is a significant character. He is a powerful and cunning leader within the Dark Eldar society.
The Warhammer 40k Dark Eldar lemon fanfiction is mainly centered around the Dark Eldar race in the vast Warhammer 40k setting. It might tell stories about their cruel and sadistic society. In these fanfictions, the authors may focus on the individual characters among the Dark Eldar, their personal quests for power or survival. Since it's 'lemon' fanfiction, it could potentially include some romantic or sensual undertones related to the Dark Eldar characters. This might involve their courtship rituals, which are probably very different from what we know in our own world, considering their dark and often violent nature.
One popular character is Yvraine. She is often depicted in fanfiction as a powerful figure leading the Ynnari. Her unique abilities and her role in the Eldar's struggle for survival make her a fascinating character to write about.