Zombies are a common element. Since Left 4 Dead 2 is all about fighting zombies, in crossovers, the zombies often play a central role. They might be different in some ways, like having new abilities if it's a crossover with a more supernatural - themed universe.
One interesting concept could be a crossover with another post - apocalyptic game, like adding characters from Fallout into the Left 4 Dead universe. They could bring their own unique skills and weapons, which would change the gameplay dynamic. For example, a Fallout character might have a Pip - Boy that could be used to detect special items or infected in a different way.
Well, it could be about the survivors from Left 4 Dead finding themselves in a world overrun by Nazi zombies. They might have to use their skills and weapons from their own world to fight these new and more menacing foes. Maybe they team up with some resistance fighters who have been battling the Nazi zombies for a while.
One idea could be a crossover with a post - apocalyptic zombie series. The characters from Left for Dead could find themselves in a different zombie - infested world and have to adapt to new threats and survivors. For example, they might encounter a group that has a very different approach to survival, like a religious cult that believes the zombies are a form of divine punishment. This could lead to conflicts and alliances as the Left for Dead characters try to find their way back to their own world.
Next, come up with a plot. Maybe the Left 4 Dead survivors are transported into the other universe, or vice versa. The plot should be engaging and give a reason for the crossover. You could start with a small event that leads to a bigger adventure. For instance, a strange portal appears in the Left 4 Dead world that sucks the survivors into an anime city filled with new threats and allies.
The main characters would be a mix of both universes. From Left 4 Dead, all four of the main survivors would play important roles. They would interact with characters from the Nazi Zombie side. It could be a group of resistance fighters led by a fearless commander. There might also be a character who is a hybrid of sorts, perhaps a survivor who was experimented on by the Nazi zombie creators and now has some unique abilities. This character could act as a bridge between the two groups, helping the Left 4 Dead survivors understand the nature of the Nazi zombies and how to defeat them.
Perhaps Voldemort somehow manages to bring the Left 4 Dead zombies into the wizarding world. The Order of the Phoenix and the Death Eaters are forced to team up to survive. The Death Eaters' dark magic might be used in new and unexpected ways against the zombies, and the Order's more defensive and protective spells would also be crucial. Harry, being the hero, could figure out a way to stop Voldemort from bringing in more zombies and find a way to send the existing ones back.
One interesting 'left 4 dead 2 crossover fanfiction' is the one that crosses it with Resident Evil. It combines the zombie apocalypse themes in a really cool way. The characters from both universes interact in unexpected ways, like the survivors from Left 4 Dead 2 trying to survive in the Resident Evil world full of different types of monsters.
Harry could also be very interesting. His experience in facing dangerous foes in the wizarding world would give him an edge. He has a natural courage that would serve him well in the zombie - filled world of Left 4 Dead. His ability to think on his feet and his connection to the wizarding world's resources could make for some exciting storylines. For example, he could use the Marauder's Map to track the movement of the zombies or call on his Patronus to fend them off in a really dark and scary situation.
From the Resident Evil franchise, characters like Leon S. Kennedy or Jill Valentine would be great. They have experience with fighting bio - organic threats, similar to the infected in Left for Dead. Their combat skills and knowledge of survival in a world overrun by monsters could blend well with the Left for Dead characters. They could exchange tactics and weapons, and together face new challenges that neither group has seen before.