There could be many interesting scenarios in such a fanfiction. Maybe 10k, with his survival skills from Z Nation, ends up in the Twilight universe and meets Bella Swan. He could help her in a different way from the Cullens, perhaps using his combat skills against some new threats in Forks.
Another idea for a fanfiction involving 10K in Z Nation is a time - travel scenario. What if 10K somehow got sent back in time to before the zombie outbreak? He could use his knowledge of the future to try and prevent the apocalypse, or at least change some things to make survival easier for everyone.
One idea could be a crossover where Naruto characters end up in the Z Nation world. For example, Naruto with his shadow clone jutsu could be really useful against the zombies. He could send out clones to distract the zombies while the group tries to find a safe place.
Well, 'The Lone Survivor in Z Nation' is a popular one too. It tells the story of a single person who manages to outwit the zombies and other dangerous groups all by themselves. The way the author describes the survival tactics is really engaging.
They could be friends. 10k, having seen so much death and chaos in Z Nation, might find Bella's somewhat normal life (despite the vampires and werewolves) refreshing. And Bella could be interested in 10k's tales of survival.
I'm not sure exactly who the main characters are as I haven't read it thoroughly. But they are likely to be a diverse group of survivors with different skills and personalities, such as a strong fighter, a smart scientist or a resourceful scavenger.