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.
A really interesting plot could be a Dark Eldar Archon making a deal with a powerful psyker from another faction. The Archon wants to use the psyker's powers to enhance his own position in the Dark City. But the psyker has his own plans and secretly starts to manipulate the Dark Eldar's internal power struggles. This would lead to a complex web of intrigue, betrayal, and power plays within the Dark Eldar society as well as between the two factions.
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.
One interesting plotline could be a Dark Eldar raiding party stumbling upon a lost Imperial relic. They see it as a valuable item to either use for their own dark purposes or sell on the black market in Commorragh. Another could be a story about a Dark Eldar wych who tries to rise through the ranks by challenging powerful rivals within her own cult.
One interesting story could be about a Dark Eldar raid on a Imperial outpost. They swoop in with their fast ships, taking the Imperials by surprise. They capture slaves and valuable resources, leaving destruction in their wake.
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.
There are fanfictions that focus on the relationship between different Eldar Craftworlds. For example, a story where two Craftworlds must unite their forces against a sudden Tyranid invasion. This involves political intrigue as well as epic battles, with the Eldar using their psychic powers and advanced weaponry in creative ways.
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.
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.
A great read is 'The Kabal of the Black Heart' novel. It focuses on a particular Kabal within the Dark Eldar. It shows their relationships with other factions in the Warhammer 40k setting, their thirst for power, and the dark magic and technology they use. This novel gives a detailed look at their military strategies and the internal politics of the Kabal.
Perhaps an Imperial Inquisitor ends up in the Star Wars galaxy and tries to manipulate the Sith and Jedi for his own ends. He could see the Sith as a useful tool, not realizing how dangerous they truly are until it's almost too late. This could lead to a three - way battle between the Inquisitor, the Sith, and the Jedi.
One idea could be a crossover with the Star Trek universe. The Tyranids could be seen as an extremely hostile alien force that the Federation has to deal with. Their bio - ships and swarming tactics would be a huge contrast to the more technological - based Starfleet.