I can give you my opinion as I solely play “Zombie/Survival Games” on steam and have one of my own designed on paper . . .
First off, you want to have a new idea. Something that has not been done yet to take the current zombie genre to the next level. If you simply just take a mixed bag of available gameplay options from other games and compile them you will have a unique game mode, but it will become tired quickly. Thanks to so many unprofessional developers in the past your zombie game will be coming out of the door with negative attitude towards it simply because “oh look another zombie shooter -_- . . .” and so having something different that is fresh will help you get past that firewall.
Next, you want to come up with an actual story or lore to the zombie infestation. So many games have this DayZ “You figure it out” attitude towards thier development and in all honesty it just makes the developer look lazy and uninspired. You want a theme to hook your gameplay mechanics to so that there is a feel of immersion more than just controlling a player with a keyboard. It does not have to be a monumental novel of a story but just a core synopsis of what the player is actually going through in the game and why. Without that people result in griefing as a natural alternative to boredom, which has been done to death in Zombie games.
When it comes to the type of gameplay versus the zombies I would suggest trying to find an even ground between DayZ and Left 4 Dead. Having a walking simulator is not only a huge waste of time and resources but never works. Players always naturally gather in the “high population” areas and so it may be wise to build your game mechanics on that assumption instead of having lots of level space with no real use. If you go the other route and choose a Left 4 Dead style gameplay than I would suggest you try to give some space and options for the player to wander outside of an “On Rail” shooter experience. A good zombie game to me is one where you have to move from resource to resource continuing your survival. A game that relies on you bunkering into a corner and just hosing down mobs of zombies can get boring very quickly just as having to wander for 30 mintues in any direction to get to the next weapoins cache can also be. So I think you get my point. You want a good middle ground between “downtime” and “oh ****” time when in combat.
Alot of the questions you have about the core mechanics such as: zombie damage, player movement, ect. are going to depend on both the story/lore of the game and how hardcore you want the experience to be. One thing I can say for certain that I despise in survival games is “fake realism”. What do I mean by that? Consider games like WarZ: They have a thirst and hunger bar that depletes in less than an hour and will instantly kill you (hyperrealism) but you can shoot a player in the face with a 7.62 round and it does like 20% damage followed by them autohealing with the press of a button (complete lack of realism). So my point here is to make sure your game theme or style MATCHES the gameplay mechanics. Do not make your game look hyper realistic and with true to life weapons if you are going to use fantasy style mechanics and likewise do not use hyper realistic game mechanics if you plan to use fantasy style art and design (Plants vs Zombies for example).
One last thing I owuld mention is to streamline the UI as much as possible. In a survival game having character control is required. If the player dies or loses because of user interface holding up thier controls it can really ruin the experience and feel like cheap deaths. Since you are working on the inventory system right now I would suggest a few things that most zombie games do not do properly. Allow the player to move even with inventory windows open and always give a “one button” exit ability so if they are engaged by zombies they can instantly get back to combat irrelevant of what UI window is open. Behing stuck in an inventiry “bag” while being shot at or eaten in a panic can really make me stop playing a game, especially if there is loot involved that can be lost (otherwise seen as wasted time).
I hope these suggestions help you draw out a sketch of where your prject is heading and what you want to produce with it. We really need some good SCARY zombie survival games out there in my opinion so having a dark themed immersion may be a design route.