Well, a cool storyline could involve the Flood from Halo and the Tyranids from Warhammer. Imagine if the Flood somehow made its way into the Warhammer universe. The different races in Warhammer would have to deal with this new, insidious threat that spreads like wildfire. And on the flip side, the psykers in Warhammer could potentially have some interesting interactions with the Halo universe's Covenant, who have their own sort of 'psionic' abilities with the Prophets and such.
Some interesting concepts are the different power sources. In Halo, we have things like plasma and fusion power, while Warhammer 40k has the Warp and its associated powers. How would these interact? Maybe a UNSC ship powered by a fusion reactor has to deal with the unpredictable effects of the Warp when it enters a region influenced by it. And then there's the concept of heroism. Spartans are known for their individual heroics, while in Warhammer 40k, heroism often comes with a heavy price and a lot of religious and ideological baggage. Combining these two views of heroism can make for some thought - provoking stories.
Some interesting concepts include the combination of the different power systems. In Halo, you have the MJOLNIR armor with its energy shields, and in Warhammer 40k, there are psykers with their psychic powers. Imagine a scenario where a Spartan with MJOLNIR armor has to face a Warhammer 40k psyker. How would their powers interact? Another concept is the cultural exchange between the two universes. The military hierarchies and honor codes in both settings could lead to some really complex and engaging storylines when combined.
The sleek, high - tech look of Halo's UNSC and Covenant ships can contrast sharply with the gothic, ornate designs of Warhammer's spacecraft and architecture. In fanfiction, authors might describe how the angular, metallic forms of Halo coexist with the spiky, daemon - influenced aesthetics of Warhammer. For example, a UNSC base on a Warhammer world might look like a strange outlier, or perhaps over time, it starts to adopt some of the local, more flamboyant design elements.
In Halo Warhammer 40k crossover fanfiction, the Librarians from Warhammer 40k are quite popular. Their psychic powers can lead to some really cool interactions with Halo's technology and characters. They can sense things in the Warp, and how would that work when dealing with the more scientific Halo universe? Cortana from Halo is also a favorite. Her artificial intelligence is something that doesn't really exist in the same way in Warhammer 40k. So seeing her interact with the various tech - priests or even daemon - possessed entities could be really interesting. Additionally, Chaos Space Marines are often used for their menacing and unpredictable nature in crossovers.
It could be the Emperor of Mankind from Warhammer 40k. His psychic powers are off the charts and his influence over the entire Imperium is vast. He could easily manipulate the situation in a crossover. However, Master Chief from Halo also has his own set of skills and experience that make him a formidable opponent.
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.
In a Halo x Warhammer fanfiction, there could be a scenario where the Flood from Halo is unleashed in the Warhammer universe. This would be a huge disaster as the Flood has the ability to infect and assimilate almost any living being. The Imperium of Man in Warhammer would have to come up with new strategies to deal with this alien threat. Maybe they would try to use their psykers to combat the Flood's hive - mind. And the Halo UNSC forces might also get involved, bringing their own technology and tactics to the fight.
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.