I remembered a german shepherd AI model in Arma II: OA so I thought this might be a possible gameplay idea:

There should be wild dogs in the game. I mean packs of wild dogs that attack the players. A player should be able to domesticate one if you kill all dogs in the pack except one and offer him food by dropping an opened can on the ground or wound him and then fix him up, so the player can become the Alpha who the dog will follow around. Should be carryable in larger backpacks to transport him through dog-inaccessible-areas like ladders.

The player should also be able to let a dog sniff on dropped items like clothing or trash left behind by another player. The dog would then be able to guide the player to the other player and pointing them or other players out by growling in their direction when they get nearby and bark when get very close. Player should be able to give an order to attack them, to sit and wait somewhere, to distract by barking, or to be quiet (except when aggro'd zombies come for the player, then they would always bark at the zombie out of fear/danger).

If wounded they should be treatable like other players (bandaging, force feed/drink and saline bags or the like).

The player should also have new responsibilities with a dog as a companion. He should have to take care of the dog by dropping opened cans of food for him otherwise he might run off. When the dog turns extremely hungry it should even attack the player again.

Domesticated dogs should be afraid of aggro'd(!) zombies tho', barking at them, keeping some meters distance to them. So the player still has to take care of those himself, especially to quiet the dog who will give away the players position by barking at the zombie. This should be nice for balancing.

Zombies should still only be interested in attacking players.

Think this is a good idea? vote up, down, sideways, whatever.