One idea could be to have Spartans from the UNSC interact with Halo's Forerunner technology in unexpected ways. Maybe they find a Forerunner device that gives them new abilities or reveals secrets about the Halo rings.
Another interesting concept is a time - travel crossover. Have UNSC forces travel back in time to the era of the Forerunners or forward to a future where the Halo universe has evolved in new and strange ways.
A great UNSC crossover fanfiction idea is to cross it with the Halo universe itself but in an alternate timeline. Maybe the UNSC discovers a new Halo ring that has different properties or is inhabited by a previously unknown species. Or you could have a crossover with the Transformers. Picture UNSC soldiers trying to figure out how to deal with giant alien robots. Also, a crossover with the Stargate universe would be interesting, with the UNSC exploring different planets through the Stargate network.
In 'halo 4 crossover fanfiction', a great crossover idea is with the Ghost in the Shell universe. The high - tech world of Ghost in the Shell combined with Halo's advanced military tech would be fascinating. You could also consider a crossover with the Metroid series, where the Spartans face off against the space - faring threats like Samus does. And a crossover with the Devil May Cry series would bring some stylish combat to the Halo universe.
Well, in some 'halo unsc becomes powerful fanfiction', there might be a plotline where the UNSC teams up with an alien race that they previously thought was an enemy. This alliance could be the key to their newfound power. They could share technology and knowledge, which leads to the UNSC becoming a more formidable force in the galaxy. Maybe they learn how to build ships with faster - than - light travel that are also more durable and better armed.
A cool concept would be a hybrid character. Imagine a Spartan who has been genetically altered with some dragon DNA. This character could have the strength and combat skills of a Spartan along with the ability to breathe fire or fly like a dragon. Or perhaps the Covenant could be interested in the dragons in the HTTYD world because they see them as a new form of powerful weaponry to add to their arsenal.
Another concept could be a story where a character from Halo, say Master Chief, gets transported to the Overwatch universe. He has to adapt to the different rules and powers there while trying to find a way back home. Along the way, he could form unexpected alliances with Overwatch heroes and learn about their world, and they could learn from his military discipline and combat experience.
Maybe the Forerunner technology in Halo could be related to some of the pre - war technology in Fallout. A story could explore how the two different technological systems interact. For example, the power armor in Fallout could be enhanced with Forerunner tech, creating a new super - powered suit.
For 'halo anime crossover fanfiction', a cool concept is to mix the Covenant from Halo with the Titans in Attack on Titan. The Covenant's alien races and their zealotry could be an interesting contrast to the humans fighting Titans. Also, think about having Cortana, the AI from Halo, integrated into the world of Ghost in the Shell. Her digital existence could bring new perspectives to the cybernetic world of that anime.
One idea could be a scenario where the Doom Slayer somehow ends up in the Halo universe. He might team up with the Master Chief to fight a common enemy that combines the demons from Doom and the Covenant or Flood from Halo. For example, a new threat that has the ability to corrupt both the technology in the Halo universe and summon Doom demons.
Perhaps a crossover where Master Chief and Ruby Rose meet. They could exchange their combat experiences. Ruby could be amazed by Master Chief's Spartan enhancements and weaponry, while Master Chief might be intrigued by Ruby's Silver - eyed power. They could then go on a mission together, with Ruby's speed and scythe - wielding skills complementing Master Chief's brute force and strategic thinking.
One idea could be that the rookie gets transported to a different Halo timeline and has to team up with classic Halo characters to face a new, powerful enemy. For example, they might end up in the time of the Covenant War and use their unique skills from their own era to help the UNSC in unexpected ways.