Many items in the game are handled as whole-by-default, without providing a meaningful way to break them down or improve them in a modular fashion. While this approach affects everything in the game, including creatures and terrain, this is most evident with items the character would use to survive: weapons, outfits, and wearable containers.
An alternative approach would handle items as inherently made of parts – parts that could be replaced, repaired, enhanced, and reused.
The whole-by-default approach requires that every item be treated as distinct and singular, with modifications being relegated to a peripheral option to be accessed via specific tools. It also implies that every possible modification be either handled by a separate recipe, of which there could be dozens for modular entities, or within the limited gun-/toolmod system, or via the same tool-based enhancement procedure (reinforcing for clothing and armor, accurizing for firearms).
Nobody wants to make 60 different tiny variations of the same modular item, especially when the changes don't fit into the gun-/toolmod system or the similarly-limited clothing mods system, the latter only providing opportunity for adding thermal padding or armor. They would be a hell to maintain even if one perseveres with their creation. Nobody wants to make these, and so none exist – and thus a self-reinforcing cycle is created.
A modular, systemic approach would implement a more approachable, expansive system that would best fit a game focused around survival and atomic operations.
The easiest item to exemplify this is a modern firearm.
Modern firearms are generally fairly simple. They follow well-established approaches to handling the mechanical side of firing a gunpowder-fueled projectile and dealing with the consequences.
Modern firearms, much like modern cars, face extensive modifications by the enthusiast community, law encforcement, and the military. This includes external mods: sights, tactical flashlights, comfortable grips, recoil-reducing stocks, – as well as internal ones: waterproofing, sensitive triggers, harder springs, firing mechanism adaptation etc..
This is justified as necessary to adapting to different combat situations: a mid-range sight would come in useful for an assault rifle if the combatant expects engagement at around 300-400m. Similarly, firefights in dimly-lit environment generally warrant flashlights attached along the firing vector of the firearm, to enable to combatant to illuminate their target clearly.
Ultimately, many firearms feature a wide variety of potential modifications, including those not intended by the manufacturer. Treating a firearm like a system of parts working together towards a particular end would enhance realism by providing the player with a set of resources to make their own weapons or enhance the ones they find on the journey.
That, along with a greater range of choices of parts, would mean adaptability, improved gunplay, a greater sense of attachment to one's gear, and deeper sense for a lived-in world where parts existed before the Cataclysm for enthusiasts to peruse.
Given that the system proposes involving a lot of firearm parts, it's worth discussing the consequences to the firearm usage simulation.
Firearms vary in their dependability based on the mechanics they employ to fire the projectile and eject its shell. Some, like the Kalashnikov assault rifle and Glock pistols, are known to be very reliable. Others, like the early M16s, have the opposite reputation. While it would run orthogonal to sanity to try and reproduce the entire mechanism, a more abstracted – but still more grounded – model could potentially be used in determing not just malfunction rates but the effects on the gun's reliability from various modifications.
Substitute, inferior parts could be made for firearms that will perform their function, but should only be resorted to in the most dangerous situations where an unreliable gun is better than no gun at all. With modern firearms, machining ought to be precise, lest all sorts of malfunctions may occur. In the survivor's world, this possibility should be kept in mind.
Superior aftermarket components would also play their part in the situation. There is a certain budget within which the firearm must be produced. Aftermarket can provide superior parts – either superior for most cases or a few specific ones – because it makes for a more affordable investment for most users, professional or recreational. Being in the US is also a benefit, given the prolific gun culture of the country. Thus, it would also be important to consider how better parts could make the weapon perform better.
Most of the stats relevant to weapon use calculations are derived from a set of circumstances, something the game already gets right in some cases. Any ballistic projectile produces a kinetic response, sure, but how much of it is effected on the user also depends on the dampening mechanisms: the grip, which helps hold the recoil off manually, the stock, which may or may not provide mechanical resistance to the shock, and the firing mechanism itself.
Taking an integrative approach to different kinds of components, taking their nature into account, would be of benefit to the depth of the simulation.
One example of that could be replacing as many of the firearm's parts with non-magnetic counterparts (plastic, ceramics, non-ferromagnetic metal alloys). This enables not only a more-subtle restricted-carry weapon (as it's no longer detected by metal detectors), but also benefits of not using iron in your weapons: for example, your weapon can no longer be taken from you by a zombie with an implanted magnet CBM, and you can use your firearm for what's been dubbed the fae run, where you can't touch anything that has iron in it.
While most clothing items are going to be mostly-whole, allowing them to have parts enables a degree of modularity even to common clothing (think coats with zipper-attachable hoods), as well as an extensive moddability.
For example, a regular t-shirt could be outfitted with a variety of pockets by way of rags, thread, and a needle, enhancing its usefulness in a situation of survival.
A more reasonable example could be leather jacket with a small-arms holster and a set of magazine holders sewn onto it and kevlar padding sewn in, in order to make it significantly more useful in combat situations.
Military and enthusiast outfits feature velcro strips that allow wearers to attach outfit details, such as a kevlar vest, a set of magazine holders, a small sheath or a pistol holster, loose-items containers, among other options. Later designs of the US Army uniform prefer buttons because velcro makes significant noise on use, which could be incorporated into the game world.
Other designs, potentially featured in survivor gear, may include belts, plastic locks, magnets, or loops to attach details, including decorative tokens to make the outfit feeling more personal.
Modern armies employ modular load-carrying systems in combat and training. Parts of these systems are even already featured in the game, but provide no way to interact with other parts, instead producing generic same-layer encumbrance issues when worn together.
In fact, a lot of military and enthusiast wearable storage features loops, velcro straps, and other mechanisms by which parts of the system could be attached to one another.
Even assuming absolute conservatism of thought beyond using the load-carrying systems exactly as intended, MOLLE (presented in the game with a single medium-volume backpack) features pouches, kevlar vests, holsters, multiple backpacks, magazine holders, hydration packs, and belts – each of which has multiple loops to strap items onto.
A more grounded example could be attaching a couple of zipties to an ordinary backpack in order to fascilitate a functional, if cumbersome, large-firearm holster. Alternatively, a more involved option would take large-bottle side pockets to render their bottoms into hatches which could be closed (to store bottles, as usual) or opened (to store rifles).
Currently, the only situations where body parts matter are randomized weapon hits, butchery (parts are generic and randomized via external lists), and, to an extent, bionics installation (body parts contain "slots" which dictate how many implants can be installed per part).
A deeper simulation of body, including mutated body parts and body structures, would be excellent – it would, for one, enable aiming for a particular body part with the weapon – but even treating body parts as parts, without going deeply into their biology, would enhance the gameplay.
Parts being parts means they could be lost to damage. A human, an animal, let alone a zombie could still function without an arm, a leg, or an eye – but the consequences of such a loss make the game more dynamic. Adapting to playing your character one-handed is challenging, and it makes for a story to tell – provided the character survives losing an arm. Afterwards, displacing your anger onto a random zombie by shooting off their legs and watching them crawl towards the barrel of your awainting gun could very well be cathartic.
Parts being parts also means they could be replaced, even if healthy to begin with. This opens up a whole sea of options for bionic, mutagenic, and substitute replacements. Allowing body parts to be manipulated would bring the game closer to the possibility of biomechanical transcendence.
See also: bionic slots → space & layers overhaul
Another important aspect of modularity is that each part exists in a reachable fashion: that is, each part could be activated or used if there's an option to do so. From the player's perspective, this wouldn't be particularly different from the way one could reach and activate items in their inventory right now, even if the item is stashed within a dozen containers, thanks to the pocket system.
This could eventually result in a unified actions menu, where everything that could be used, can be used within the frame of a single menu, navigable by category, cost, and target item. Distinct categories – like mutations, bionics, and inventory – would still retain their own menus, if because they'd still provide valuable options for examination and extended actions.
The wider range of usable things in the game could necessitate an expansion of the hotkey system, in order to incorporate all possible options for power users without excluding regular players. One possible solution could be hotkey sequences: pressing one key to activate a mode of operations (e.g. "display bionics menu"), then pressing another to produce an action within said mode (e.g. "activate bionic XYZ" or "show information on currently-selected bionic").
See: Xah Lee's commentary on key chords and sequences
Treating entities as consisting of parts allows "composing" items and creatures in ways that go beyond crafting and construction.
Composability enables spawning entities of randomized customizability, further enhancing spawn of items. Things that can consist of multiple parts – e.g. a ballistic vest with insertable plates – may now be spawned with a random number of components, in random states of functionality. Firearms could spawn with different mechanisms and modifications, as dictated by the spawn lists themselves. Melee weapons could spawn slightly damaged or already modified to the previous user's needs – imitating more verisimilarly the history of the fictional world.
Applied to creature spawn, this would enable spawning monsters missing limbs and crucial sensory organs, with perhaps a trace of what had happened to them in their previous (non)existence. This again serves to prop up the believability of the world slightly: even though the player knows perfectly well it's all randomly-generated, they wouldn't help but marvel, as one does when reading a book or watching a film, to the detail with which this fiction is produced.
One interesting aspect of the latter comes up when discussing the more-bizarre creatures, someone so affected by the mutation they no longer represent the humanoid class of creatures. Some of them then might spawn with a random set of limbs – and organs, if full body simulation is to be implemented – without following a mutation line. These kinds of creatures could end up being harmless or most dangerous – something that adds to randomness without turning it into chaos.
See also: building blocks
Specific interactions and cases don't matter as much as having a system to enable them. Community contributors and modders would take care of content.
Enabling creativity for players and modders is more important than expanding content base. Fascilitating ingenuous interactions and choices for a game so focused on depth of interaction would only enhance the gameplay.