Another interesting concept could be a crossover in which the technology from TF2 is integrated into the Overwatch universe, or vice versa. For example, the Medic's healing technology being used to enhance the Overwatch medics' abilities. This could lead to some really cool character interactions and story developments as they learn to work with new equipment and each other.
A great concept could be a crossover tournament. The two teams, TF2 and Overwatch, compete in a series of challenges. These challenges could include things like a sniper - only round where Widowmaker from Overwatch and Sniper from TF2 go head - to - head. Or a melee combat round with Reinhardt and the Demoman. It would be a chance to explore the different abilities and personalities of the characters in a competitive setting.
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.
Mercy from Overwatch and the Medic from TF2 would be likely allies. They both have a strong focus on healing. Mercy's ability to fly and revive teammates could be well - complemented by the Medic's Ubercharge ability. Another pair could be Tracer and Scout. They are both fast - moving characters. They could use their speed to scout out areas, steal intel or make quick strikes against the enemy.
A more complex plot could involve a mysterious force that is causing glitches in both worlds. Some characters from Overwatch and TF2 have to work together to figure out the source of this force. For example, Tracer from Overwatch and Scout from TF2 could use their speed to scout out the affected areas while their respective teams of engineers, like Torbjörn and Engineer from TF2, try to fix the glitches in the technology.
I think Mercy from Overwatch and Medic from TF2 could be great allies. They both have a strong focus on healing and could share medical knowledge and techniques from their respective universes. Mercy's ability to fly and revive teammates could be combined with Medic's ÜberCharge in interesting ways during battles.
Mercy from Overwatch and the Medic from TF2 could be the first to interact. Given their roles as healers, they would likely be interested in each other's medical techniques. They might start by comparing notes on how to heal their comrades more effectively, and this could lead to a deeper understanding and cooperation between the two groups.
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.
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.
Social media platforms can be useful too. On Twitter or Tumblr, there are groups and hashtags related to TF2 and Overwatch fanfiction. By following these hashtags or joining relevant groups, you can discover new fanfictions shared by other fans. Additionally, some fanfiction authors promote their work on these platforms, so you might find some hidden gems.