Well, an interesting concept might be the crossover of the ninja villages in Naruto with the military bases in Halo. Imagine Konoha being like a UNSC base, with ninjas using Halo - style dropships for missions. And the idea of Naruto characters using Halo armor for protection while still maintaining their ninja - like agility would be really cool too.
One interesting concept could be the crossover of technology. For example, how the biotic powers from Mass Effect might interact with the advanced weaponry of Halo. Maybe a character from Mass Effect using a Halo energy sword in a unique way.
One interesting concept could be the crossover of Halo 5 characters with characters from other science - fiction universes. For example, seeing Master Chief interact with characters from Star Wars. It could lead to epic battles or unexpected alliances. Another concept might be the combination of Halo 5's technology with that of a different game world. This could create unique weapon combinations or new ways of traversing the game environment. Also, crossovers can explore different cultures within the Halo 5 universe and how they would react to an outside influence.
Another concept is the crossover within the Halo universe itself. Different Halo timelines or story arcs being combined. For instance, bringing characters from the original Halo games together with those from the expanded universe novels in a new adventure that fills in gaps in the overall Halo lore.
Well, in a Halo x Naruto fanfiction, there could be a scenario where the Halo's Covenant discovers the ninja world. Naruto and his comrades might have to learn about the Halo technology quickly to defend their world. And at the same time, the Halo characters could be intrigued by the chakra system and the unique jutsus of the ninjas. For example, Master Chief could be in awe of Naruto's Rasengan and try to find a way to incorporate that kind of power into his own combat style.
In this type of fanfiction, the concept of mixing the grittier, more militaristic feel of Halo with RWBY's more fantastical elements is really engaging. We could see UNSC soldiers trying to adapt to the Grimm - filled world of Remnant. And perhaps the characters from RWBY learning about the Forerunners and their technology. It could open up a world of possibilities for epic battles, new friendships, and the exploration of different values and ethics from both universes.
Well, a cool idea for a Halo Pokemon crossover fanfiction would be if the Master Chief discovers a Pokeball during one of his missions. When he opens it, out comes a Charmander. The Chief then has to figure out how to use the Charmander's abilities to his advantage in the Halo battles. Or perhaps the Covenant discovers Pokemon and tries to use them against humanity, leading to a whole new kind of warfare where trainers from both sides battle it out with their Pokemon teams.
One interesting concept could be the interaction between the Halo and Flood elements. For example, how the characters from Halo deal with the overwhelming threat of the Flood. Maybe there are unique alliances formed within the fanfiction that wouldn't happen in the original Halo lore. Another concept could be the exploration of different Flood forms and how they adapt to the Halo universe's technology and environments.
One interesting concept could be the backstories of original characters (OCs). For example, an OC might have a unique origin related to the Forerunners. Another concept could be OCs in different squad formations, like a group of OCs who are misfits but come together to achieve great things in the Halo Reach universe. Also, OCs with special abilities that are not common in the normal Halo lore, such as an OC with the power to control a new type of Covenant technology.
One interesting concept could be the Forerunner's energy manipulation. In fanfiction, they might be shown to have advanced ways of harnessing and using energy for various purposes like powering their massive structures or their advanced weaponry.
One interesting concept could be the combination of Halo's advanced alien races like the Covenant with Starcraft's diverse species such as the Zerg. For example, imagining how the Covenant's religious zealotry would interact with the Zerg's hive - mind instincts. Another concept might be the technological exchange. The energy shields from Halo could be incorporated into Starcraft's units, or vice versa, like Starcraft's psionic technology being used in the Halo universe. It could also be fascinating to see characters from both universes meet, like Master Chief and Jim Raynor teaming up to face a common threat.