I want a straight wave-based zombie game with enough content to not become a drag.
Kinda like Call of Duty zombies mode (don't leave yet, please) except better in every way (told you not to leave).
It's got enough content and large enough maps to keep you going, as well as having systems in place to get, upgrade, and customize weapons.
Waves of various undead, marauders, and mutated come after you, as well as random events that can help or hurt you, that can come at any time at all. Let's say we're playing on a sea-side map, the map is a large boat with a few cargo containers strewn over it, the cargo loading area, and the surrounding complex. Enemies on this map aside from the basic ones are aquatic in nature, that climb up and onto the map from water.
Weapons are also thrown about, but they aren't quite the same as in other games. Firstly:
Aside from the pistol you start with, weapons spawn in with random durability, ammo amounts, and mod locations. You might find a shotgun in relatively good condition (probably won't jam or misfire), with all possible mod slots open, and a full extra ammo box, or you may find it almost unusable, with no room for mod slots, and maybe a slug or two left.
You can also find attachments, as well as materials, to make other weapons and attachments-
Let's say you find a shotgun with all mod-slots open, but with questionable durability and a low amount of ammunition. You've found a few things that could be useful for it, such as dart shells, a foregrip, and a wide, open, no-zoom sight. To actually put any of them on, you have to use a workbench, which takes time and makes you vulnerable. You start to modify the shotgun between waves.
You're able to put on all the attachments easily, and then you work on the durability. You can find repair kits sometimes (rare spawns) that will fix up a weapon to full durability (guaranteed not to fail), but otherwise, you'll have to spend time trying to fix it up. There's a chance that you'll do so well, and in a short time you'll fix up a lot of the durability bar, and there's a chance youll mess it up completely and damage or break the gun. All of this takes time, and the amount of time you get between waves is determined by how well you did on the last wave. Take a long time and intermission will be a short time, not allowing you to get much done, finish a wave quickly, and you'll get a chance to relax.
Random events can be good or bad, and happen at any time- anywhere from causing a large amount of ammo to spawn somewhere on the map (say, a humvee crashes through a map edge, and explodes), or something bad (such as a sudden fast zombie swarm).
All attachments can be used on all weapons. Find a ballistic scope? Go ahead and put it on your shotgun, if for some reason you want to.
Looting areas is also important, to find things in a Last of Us style. You can improve a grip with cloth, reload shells with gunpowder (fairly rare) and other materials (any of them, though a good deal of combinations are useless- cloth shells do absolutely nothing). Some combinations are able to be crafted on the fly- molotovs, for instance.