A possible plotline is that a mysterious force brings Riddick and Daryl together to find a cure for the zombie virus. Riddick's physical abilities and Daryl's knowledge of the local area combine to lead them on a dangerous adventure. They encounter various obstacles, including other survivor groups who want the cure for themselves.
It could be an exciting crossover. Riddick is known for his survival skills in harsh environments, and Daryl from The Walking Dead is also a great survivor. In the fanfiction, they might team up to fight against the zombies in The Walking Dead universe. Maybe Riddick's unique fighting style and his ability to see in the dark would give them an edge in battles.