One great Halo and Star Wars fanfiction is 'The Spartan in a Galaxy Far, Far Away'. It combines the combat skills of the Halo Spartans with the epic space opera setting of Star Wars. The story follows a Spartan who somehow gets transported to the Star Wars universe and has to adapt to the new technology and cultures there.
First, know both universes well. You need to understand the lore, characters, and technology of Halo and Star Wars. For example, in Halo, the UNSC has specific types of weapons and ships, and in Star Wars, the Jedi have their own code and powers. Second, create an interesting plot that combines elements from both. Maybe a Halo character has to retrieve a MacGuffin in the Star Wars universe. Third, keep the characters in - character. A Spartan should act like a Spartan, and a Jedi should act like a Jedi.
Yes, there are some great Halo Reach Star Wars fanfictions out there. You can find them on various fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Halo Reach Star Wars' in their search bars, and you'll be presented with a list of stories that blend the two universes in interesting ways.
There's a fanfiction called 'Crossed Fates in the Stars'. In this one, the Spartans from Halo and the rebels from Star Wars Rebels meet. The story is detailed and full of action. It shows how the different technologies and cultures interact. For example, the energy shields from Halo and the lightsabers from Star Wars create some really exciting fight scenes. It also delves into the characters' moral compasses and how they deal with the new situations they find themselves in.
One idea could be a crossover where the Rebels' technology somehow gets mixed with Halo's. For example, the Rebels might discover a Halo ring and try to figure out its purpose while also dealing with Covenant forces. Maybe Ezra could use his Force abilities in new and unexpected ways against the alien threats.
Another theme is the clash of cultures. The militaristic and often post - apocalyptic setting of Halo is very different from the more utopian ideals in Star Trek. Fanfiction often explores how characters from both universes deal with these differences. For example, in some stories, the Halo characters might be shocked by the Star Trek's Federation's peaceful approach to resolving conflicts. And the Star Trek characters could be in awe or horrified by the more violent nature of the Halo universe at times. It makes for really interesting character dynamics and plot developments.