One interesting plot could be a female Boomer who has some sort of unique ability that the survivors don't expect. For example, instead of just vomiting on them, she can send out a sonic wave that disorients the survivors for a short time. This would add a new twist to the usual encounters in the game.
You should also consider her backstory. If she was a human before, what was her life like? What led to her transformation? By answering these questions, you can create a more complex and interesting character. For example, if she was a scientist before, she might retain some of her intelligence and use it in her actions as a Boomer.
One possible plot could be Percy's struggle for survival in a harsh and unforgiving environment. Maybe he has to find food and water while fending off dangerous creatures. Another plot could involve Percy trying to figure out who left him for dead and seeking revenge.
One interesting plot could be that Bubbles and Boomer find themselves in a parallel universe where their roles are reversed. Boomer has to be the good guy and Bubbles is a bit more mischievous. It makes for a really fun and different take on their characters.
One possible plotline could be Naruto finding unexpected allies during his time left for dead. Maybe some characters who previously had no connection to him come to his aid out of a newfound sense of justice or a change of heart.
Another plotline might involve Naruto's inner struggle and growth. He could discover new powers or a deeper understanding of his ninja way while on the verge of death. He might have to rely solely on his wits and the training he's had in the past to survive. For example, he could use his shadow clones in more creative and strategic ways to defend himself and find a way back to his village.
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.
One interesting plot could be about a group of survivors forming an alliance in a way not shown in the original series. They might trade resources and skills in a unique manner to fight off the walkers. For example, some survivors who are good at foraging for food in the wild team up with those who are excellent at building fortifications.
One idea could be a story where Bubbles and Boomer go on an adventure in a magical forest. They encounter strange creatures and have to work together to find their way out.
I'm not sure there are many well - known official stories in such a specific 'left 4 dead witch pregnant fanfiction'. However, some might imagine how the situation of a pregnant witch could change the dynamics of the game's world. Maybe it could lead to a more complex character arc for the witch, like her becoming more protective or having new powers related to motherhood.
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.