Character development is key. If you're bringing in characters from different franchises, make sure they stay true to their original selves while also growing in the new environment. For instance, if Captain Kirk enters the 40k universe, he would be shocked by the brutality but also use his wits to survive. You also need to think about how the different powers and abilities will interact. In a crossover with Harry Potter, how would a Space Marine react to magic? There are so many possibilities to explore.
One great 'Warhammer 40000 Crossover Fanfiction' is the one where it crosses with Star Wars. It's really interesting to see the Imperial Guard facing off against Stormtroopers. The different combat styles and ideologies make for a great read.
The use of miscommunication is great too. Imagine a group of Imperial Guardsmen trying to communicate with an alien race that has a completely different language and cultural understanding. The guardsmen's attempts to be diplomatic while the aliens are completely misinterpreting everything can lead to some truly laugh - out - loud moments. Also, the cultural differences can be exaggerated for comedic effect, such as the aliens thinking the guardsmen's helmets are part of their mating rituals or something equally absurd.
One idea could be a crossover between Warhammer and Lord of the Rings. Imagine the Space Marines facing off against the Orcs of Mordor. Another could be a Warhammer and Star Wars crossover, like the Imperium of Man encountering the Galactic Empire. Or perhaps a crossover with the Witcher, where Witchers team up with Warhammer's warriors to fight common foes.
A major theme is the exploration of morality. In 'Worm', the concept of 'parahumans' and their actions can be morally ambiguous. In Warhammer, there are also complex moral situations. When the two universes cross over, these moral concepts collide. Authors often use this to explore how characters from different backgrounds with different moral compasses interact and make decisions in new and difficult situations. This can lead to some really thought - provoking storylines that go beyond just the action and adventure aspects of the crossover.
One great aspect of 'Naruto Total War Warhammer Fanfiction' could be the unique character combinations. Imagine Naruto characters in the Warhammer universe, like Naruto using his Rasengan in the midst of a Warhammer battle. It would be a spectacle of different powers and fighting styles colliding.
One great 'Warhammer 40k Emperor Crossover Fanfiction' is the one where the Emperor crosses over into a modern Earth setting. It shows how his powerful psychic abilities and leadership skills shock the modern world.
One possibility in this fanfiction could be that the female Harry Potter brings her magical skills into the Warhammer universe. Maybe she uses her magic to combat the dark forces in the Warhammer world, like the Chaos legions. Her knowledge of spells and potions could give her an edge in battles and interactions with the various races in that universe.
One common theme is the clash of different cultures. The very different races in Warhammer and Mass Effect coming together often leads to interesting cultural exchanges and conflicts.
One great example is 'The Orc and the Elf's Unlikely Adventure'. It has the orcs and elves in a really funny situation where they have to team up against a common, yet absurdly comical, enemy. The way the author plays with the traditional Warhammer characteristics of these races and twists them for comedic effect is brilliant.