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 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 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.
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.
A cool plot for Warhammer Fantasy Crossover Fanfiction could be a crossover with the Final Fantasy series. Picture a Warhammer elf meeting a Final Fantasy summoner. They could exchange knowledge about different magical creatures. Also, a crossover with the Naruto universe would be interesting. A Warhammer Chaos warrior might find himself in the Hidden Leaf Village and cause chaos (literally) among the ninja. Or, a crossover with the Witcher world. Geralt could team up with a Warhammer Witch Hunter to hunt down monsters that are a threat to both their worlds.
One idea could be that the Soul Reapers from Bleach are transported to the Warhammer universe. They might initially be seen as strange entities, and their spiritual powers could be both a mystery and a threat. Maybe they get involved in the battles between different factions in Warhammer, using their zanpakuto and kido to fight against the chaos forces.
One idea could be that Naruto characters are transported to the Warhammer universe. For example, Naruto himself could be mistaken for a powerful chaos warrior due to his chakra. He might then have to prove that he's not a threat while also trying to find a way back home. Another plot could involve the shinobi clans of Naruto being compared to the different factions in Warhammer. The Uchiha with their Sharingan could be seen as having a similar level of mystery and power as some of the Warhammer sorcerers.
One idea could be that the Infinity Stones are somehow connected to the Warp in the Warhammer universe. Maybe Thanos discovers a way to tap into the power of the Warp through the Stones, which brings the heroes of Marvel into conflict with the daemons and factions of Warhammer.
One idea could be that the Doom Slayer is transported into the Warhammer universe. He might start by facing off against hordes of Orks in a brutal melee, using his signature weapons. His superhuman strength and unwavering determination would make him a force to be reckoned with among the various factions.
Another plot could be a crossover where the Skaven meet a group of powerful mages from another universe. The Skaven, being cunning, would initially try to deceive the mages and use them for their own gain. But as the story progresses, they find out that the mages are not so easily fooled and a battle of wits and magic ensues.