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.
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.
Chief Master Sergeant John - 117 from Halo is often a popular character in these crossovers. His super - soldier abilities and leadership make him a natural fit in the chaotic and dangerous Warhammer 40k universe.
In Halo x Warhammer 40k fanfiction, there might be crossovers where the Forerunners' technology from Halo is discovered by the Imperium of Man in Warhammer 40k. This could lead to a power struggle as different factions within the Imperium vie for control of this new and powerful technology. Also, the Flood from Halo could pose a unique threat to the Warhammer 40k universe, and seeing how the various races in that universe would deal with it would be really fascinating.
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.
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.
One interesting concept is the crossover with other popular sci - fi universes. For example, a crossover with Star Wars could have the Imperium of Man facing off against the Galactic Empire. Another concept could be the combination of different Warhammer 40k factions in unexpected ways within the fanfiction. Maybe the Space Marines teaming up with the Orks against a greater threat.
One interesting plot could be Naruto's ninja skills being used in the grimdark universe of Warhammer 40k. For example, his shadow clone jutsu could be a great asset in battles against the various alien threats in 40k. He could create hordes of clones to overwhelm the enemy forces like the Tyranids or Orks. Also, his ability to sense chakra might be similar to the psykers' ability to sense the warp, which could lead to some unique interactions with the Imperium's psykers or the Chaos sorcerers.