Maybe make it so it is like a dice role for someone to turn into a Zombie or make them have a certain amount of time before they explode after they turn Zombie!

Have different strands of infections people suffer if they get attacked or have different types of Zombies that infect people in diferent ways?

I was think like have Zombies classes like leader Zombie that can give you all kinds of deseases that you get and you could maybe infect your team mates!

Have a healer class that can cure different illness as well maybe?

Have zombies that can pick up weapons or objects and use them against you like an arm or leg!

Don't get rid of Fred make him the focal point again just now have him bigger and wider have the ground shake when he is around have it so if you get to close he can drag you to him and give him lickers or something that tries to protect him!