There has to be threat of characters dying. We can't all have characters that live through the whole thing. The power to kill off characters lies with the GM. 

Ex would be there are 14 characters and 7 of them are out on a dangerous run. The GM then puts the name of the seven characters in a randomizer and the character that is picked dies IC. Not like a snap they are dead for some strange reason but there is a reason. The Gm can either come up with the reason and the player has to come up with a way to input said reason or the GM can give the player a certain number of posts to come up with a reason for their character's death. Then, after the character is dead they are allowed to create another and find a way to introduce that character to the existing cast. Something like that. 

Diversity is also a very important thing. Like a previous post said, not everyone can be a badass. We have to have some "weak" characters, and others with some sort of beneficial trait. 

Take some inspiration from the Walking Dead if at all possible, there is a reason it is still running. The infected aren't the main threat, other people are. Sure the infected are still a threat, but they have more problems with other groups of people than they do with the walkers.