Go study Resident Evil Outbreak game styles.
1) Zombies should basically feel like they are slow, but tough. They should be able to withstand most attacks unless you are skilled or cunning. A zombie's speed would be 1/4 the speed of a player, but they make up for it with a slow but powerful grapple and large numbers (in open public places). If a zombie grabs you, they'll try to bite you. Other zombies can't interfere but if you break out of it, you'll still have to deal with the rest of them, thus you get blocked off. Zombies needs to have a "wandering" AI so they'll move about randomly, rather than standing in a crowd and reacting when they noticed players.
2) Survivors can shove zombies away, by running by them. Zombies can only grab you if your standing or moving at a walking speed or less. Survivors should have a "Action" ability, which allows them to do various actions with one button. How you can manage this, is having the name of said action "appear on top of your survivor" when your interacting with something. Example: You are close to a zombie, hitting X (action button) will use up some energy, but you will shove the zombie back, stunning them for a short duration. If you have a melee weapon equipped, X will do a execution attack the closest zombie, costing some points.
3) Weapons in the game should feel scarce, but killing zombies should still be an option, and beating the game without killing anything is also a option.
4) Puzzles should be incorporated into the game as mini-games that pops up as a interface. Say you want to pick-lock, then developing a interface that tests the player's skills is important too. Some puzzles require a item to use, and some requires multiple players.
5) Stealth is a option, and you should enforce that, by rewarding the players with saved ammo, and less damage.
6) Have the game contains mystery on the location of the area. Like secret locations that are only found if you get specific items and beat certain puzzles.
7) Bosses takes on a form of a patterned monster, where they will slowly chase after the player, from room to room, and try to kill them in a slow but immersive manner. At some point, you'll trigger a boss battle where you must kill him to attain a item, but you get a choice on how to do so (the game should allow you to indirectly kill enemies, like trapping them in a machine that kills them).
It's all close to the Resident Evil style btw.