To start an Overwatch crossover fanfic, you should begin by getting really familiar with both Overwatch and the other universe you're crossing it with. If it's a crossover with the DC Universe, study the powers, personalities, and backstories of the characters from both. Then, come up with a conflict that would require both sets of characters to work together or against each other. For instance, a powerful villain from DC has stolen some crucial technology from Overwatch. Outline your story, including the main events, character arcs, and how the crossover will end. Don't forget to give your story a unique title that reflects the crossover.
One interesting idea could be an Overwatch crossover with the world of Harry Potter. For example, Tracer could be a student at Hogwarts, using her time - traveling abilities in magical ways within the school. Another idea is a crossover with the Star Wars universe. Maybe Reinhardt as a Jedi, with his rocket hammer being a unique lightsaber - like weapon. And a crossover with the Lord of the Rings, where Orisa could be a guardian of a certain part of Middle - earth, protecting it from the forces of evil with her shield and gun.
Another possibility is that there's a rift in time or space that brings the two universes together. The TF2 and Overwatch characters find themselves in a strange new world that's a mix of both of their own. They have to figure out how to get back to their respective universes while also dealing with local threats. Some characters might be tempted by the different cultures and powers they encounter, which could cause internal conflicts within the combined group.
Another interesting concept could be a character swap. Imagine Tracer from Overwatch in the Halo universe, using her time - jumping abilities in the middle of a large - scale Halo battle. Or a Spartan in the Overwatch world, trying to adapt to the more urban and team - based combat scenarios there.
Well, a cool concept for an Overwatch RWBY crossover fanfic would be to focus on the technology aspect. Overwatch has a lot of advanced tech, and RWBY has its own unique Dust - based technology. The fanfic could explore how the two types of technology interact. For instance, Mercy's healing staff could be affected by Dust in the RWBY world, or Torbjörn could use RWBY - style materials to upgrade his turrets. And of course, the character interactions would be great. Winston and Zwei could be an interesting pair to write about.
A crossover could involve the characters swapping abilities. Imagine Tracer from Overwatch having the ability to call in a Titan like in Titanfall 2. Or a Titanfall 2 pilot having the ability to use some of the support abilities of the Overwatch heroes, like Mercy's healing. This would create really interesting and unique combinations in combat scenarios.
Mercy from Overwatch and Cortana from Halo might have an interesting interaction. Mercy is a healer and Cortana is an AI. They could discuss the ethics of artificial intelligence and how Mercy's healing technology could potentially be applied in the Halo universe, or vice versa how Cortana's knowledge could enhance Overwatch's medical technology.
One of the most interesting pairings would be Reinhardt and Pyrrha. Reinhardt is a knight - like character in Overwatch, and Pyrrha was a very skilled fighter in RWBY. They both have a strong sense of honor. Reinhardt could teach Pyrrha some of his combat techniques from the Overwatch world, and Pyrrha could show Reinhardt some of the unique fighting styles in RWBY. Also, Widowmaker and Weiss could be an unexpected but fascinating pairing. Widowmaker's cold - blooded assassin nature could contrast with Weiss' more noble upbringing, and they could learn from each other in terms of combat and strategy.
Medic from TF2 and Mercy from Overwatch. They're both healers, but they have very different approaches. Medic is a bit more... unorthodox with his healing methods, like using his Medi - Gun. Mercy, on the other hand, is more angelic and serene. It would be fascinating to see how they interact, perhaps they could learn from each other's techniques and develop new ways to heal the combined team during a tough battle.
One great Overwatch Bastion crossover fanfiction is 'Bastion in Another World'. It has Bastion being transported to a fantasy realm. The author does an amazing job of describing how Bastion's mechanical nature interacts with the magic of the new world.