I'll try to stay in the "suggestion" mood of things, but since I have myself dabbled some in game design, even if simply as a half assed advisor (my brother has been working on and off on a project of his and having his setbacks, too), I might end up just saying what I would do. But I guess that's what suggestions are all about, so here goes :
About the PvP system
There should be at least 3 server options:
Relaxed PvP: a flag system, so that a characters can choose or not to be flagged for PvP. There's a X minutes delay to turn off that flag, configurable.
Hardcore PvP: It's always on, babay !!
No PvP: Oh no, you don't !!
Then again, in Alpha it might start off as being able to support only a small amount of people with no search engine for servers, so friends only mostly. PvP should always be on, will allow us to test the combat system and its bugs right off the alpha ?
About the combat system, if you don't mind ?
Talking about PvP leads to talking about the combat system. Supplying a combat system for a zombie apocalypse survival game is quite ambitious, to me. You got to take care of melee with a wide variety of weapons, so you also got to take care of many melee "fighting styles" and animations and stats, etc. It's always nice when a combat system feels realistic. Plus PvP requires balance through it all. If guns are to be rare, maybe one should be able to turn the tide of a battle quickly. Or maybe there's always the need to be ready for melee since guns have a tendency to jam ?
But that makes me wonder about other kinds of ranged weapons ... more primitive ones such as bows, crossbows, slingshots, spears, rocks, bollas (really ?). Implementing those would add a whole new dimension to the game and really work towards setting it apart from all other games. Something akin to a first person medieval combat games ? That makes me think of shields ... I'd use one.
Sorry if I ended up with questions here, I got carried away :D
About cooperative survival and character design et al
From a design point of view, this again seems very complicated. Especially since it also depends on if you chose to focus more on a single player VS multi player point of view. It's no secret than a game being either single or multi player, or balanced to allow both equally, can greatly affect gameplay choices ...
So this leads us to the core of the matter.
Cooperative survival gameplay is closely tied in to choices in character design, too. In that kind of coop, a character's uses in the game is greatly determined by his innate or learned abilities. Cooperative play will be all about players bringing something needed or something special to the table.
We will need crafters of all kinds and gatherers of all kinds. We will need people who are knowledgeable and smart, maybe trainers. We will need people who are better at fighting than others. Others that are better at NOT getting into fights, getting out of a tight spot quickly or getting into one undiscovered.
More precisely, skills such as hunting, fishing, cooking, wood working/gathering, leatherworking, tailoring, metal working, mechanics, sciences (could be sub categorized) and all the more "combat" and/or "special" skills.
Survival skills are a big part of such coop gameplay. Subcategorizing survival skills into many more specific ones can be a good way of balancing single vs multi player. By forcing players to dump points in more skills by giving more skills, and by giving skill points more slowly on multiplayer servers you balance both the need for cooperative survival depending on server size and make it possible to have an always alone kind of game. On PvP servers, this could be managed differently, giving a bit more rewards for lower skills since the focus would be more on combat ? Or not ?
The downside of giving as many skills that are implementable as possible is it force implementing so many things (yeah, thats sounds logical too!). But that's what the most ambitious zombie survival game ever has to achieve, in my opinion.
Ability to make a child character ? Knowing it couldnt age and would logically have lower physical stats, maybe it could have a few special perks, like being able to fit in smaller spaces, better dodge hits, etc.
World generation / how the TDL world is made
I always have had many ideas for games, but never really looked into the more technical side of things. So what I'm gonna get out of my hat might not even be possible, but stay with me ;-)
We are to be able to exploit and mold and affect the world around us if we are to have the True Sandbox Zombie Survival Experience(I dub thee TSZSE) where we can do what we really would do if real life became the zombie survival game (sppoky).
So that makes me think we need in game tools/actions that allow us to simulate, with animations, sounds and fun camera angles and rotations (heheh, just goofing around) the feel of having affected the game world while giving us the possibility to affect it more deeply than ever. It could go as far as having actions that can let us break down flimsy walls for a hasty retreat. Or even better, make a flimsy wall (or only parts of it) into a tough one and vice-versa.
A more concrete example of what I'm trying to say is a character skilled as a mechanic would be able to take a toolbox and spare car parts to a broken truck, see himself using the toolbox on the vehicule and have a truck that can now be driven if it has fuel.
Maybe it could be as complex as being able to dig tunnels/trenches around a compound, being able to fill it with water and zombie crocodiles and laugh like a mad scientist all along ?
PS: I now realize that since I saw your project on Kickstarter it had me thinking. Thank you for giving me the opportunity to get it all out Zag ! I'm tired.
PPPS: Longest post ever :S