What's the problem?
Ah, so, allow me to elaborate with examples. (If you were actually trapped in a room with me, a whiteboard, and a projector, you'd really regret this.*)
I love designing mechanics, the more complex and obscure the better. As an example, the pinnacle of systems that convinced me that I needed to limit myself. At the time everything seemed rather simple, it was a straightforward calculation that I could do in my head in less than a minute, but it adds up. It's an integration to calculate the damage, another pair of rolls to calculate the exact cost of the ability (which can explode into a self-debuff roll if the cost exceeds reserves or is botched), a quick accuracy check, followed by a run through of the target's shields (inc. Hard damage absorptions, soft absorption, any armor piercing/shredding specials), armor (inc. everything used in shields + more), and misc. defensive abilities, followed by updating the variable statics (health,stamina,energy,armor,position,buffs,debuffs, etc) for all involved.
It's the kind of thing that works well in a programmed loop, but tends to wear down the human psyche rapidly and frustrate players when the DM keeps forgetting things. No part of the above is particularly weird, it's just really detailed. I still LIKE that system, I think it's a system that allows you to play a lot with the nitty-gritty of what separates individual methods of dealing and preventing damage and I have fond memories of the times when I was well rested enough to force it into functionality by sheer willpower and enthusiasm. If you'd like to see a DM flame-out from a mixture of life and trying to drag along a bad system, you should read my D-22 game. It's 50% game and 50% cringe worthy life issues on my part. It's rather an embarrassing pair of stone legs to leave in the sand, but it's mine.
On a slightly less personally depressing note, let's consider the sort of zombie-ish game that my opioid addled brain thought was a great idea. Right off the bat from my earlier descriptions we have a couple decent hooks for mechanics. We have genemods, and the implication that they're controlled uses of this Saigai (Bad romaji* for Calamity) strain which apparently fucks animals up horribly. Players should be allowed to have some fun with this, so there are a couple of options...
1. [Totally Anal] We provide a list of available slots and an exhaustive list of available mods.
2. [Mostly Anal] We provide a list of available slots and example genemods, then let the players write their own subject to individual approval.
3. [Probably Normal] We provide a list of available slots and ask the players for a character idea (not a description) and a list of animals. We then derive some genemods semi-randomly and check to see if the players want to re-roll that.
4. [Laid back] We let the player completely describe their characters and then make shit up in order to fit the description.
5. [Feckin' Australian M8] We let the players write their own genemods, pop a beer, and just go with it.
Now, as with most things, totally anal is not often a good option. Exhaustive lists, I've learned, are bloody exhausting. Typically ending up with several hundred entries that no one really cares to read, but will still frustrate them when they can't find what they want, or find that you've left out pangolin type genemods.
Personally I'm caught between normal and mostly anal. Mostly anal let's me ensure that people have their own say in what they get, but also keeps me in control of what actually goes through. Mostly anal forces me to adapt a bit more and make a lot of rule judgments on the fly, but that's good for expanding the system. Probably normal takes away a bit of fine control from the players in exchange for screwing about with broader concepts and reducing mechanical frustrations between GM and player.
The latter two options shift control to the players, leaving it up to the GM to ensure that the characters are still mechanically viable while allowing them to do whatever the hell they please with their abilities. This is harder than it looks.
Regardless of the above, we still need the underlying structure to support the game. I like structures that are unique and fun to play with, and playing as infected monstrosities is a good first step. So, since I've been on a DSIII kick, and it was hinted at the intro, let's make it hard as fuck to actually die. Not hard to get injured, not hard to get broken and mangled, just hard to die.
So, how about a health system where your max health represents a chance to actually kill you instead of a hard limit. Let's say you have 30 health. Here that would mean that if you've taken 30 damage an opponent can go in for a killing blow, which would have (completely arbitrary dice number) a 1 in 6 chance of killing you. If the d6 comes up 1, you die. If it comes up anything else, you gain back 30 health and the fight continues.
What's fun about this is that we can also introduce a system of status effects that are also based on damage. So, same idea as the kill. All status effects have a certain damage threshold that they need to gain one chance to inflict that effect. You can stack as many chances as you want (up to 6 in the case of the d6) to get a higher percentage chance to inflict the debuff. Whether or not the debuff succeeds, the target gets however much damage you gambled back as health. Let's say we have a status effect of Bleed. Bleed I requires 2 points of damage to get a chance to proc. So, if an opponent is down 30 HP, you can either gamble a 16% chance for an instant kill (with an 84% chance to give a full heal instead), or give them back 12 HP for 100% chance to inflict bleed. (which might only do 1d4 damage per turn, but that shit adds up.)
On the surface, which is all I really have, I like it. After a couple of combat encounters where players get limbs removed, bones shattered, and repeatedly set on fire, it should really set the mood of constant and inevitable decay (perhaps I drunk a little deeper from the dark souls cup than I intended) where death seems preferable to the mountain of debuffs piling up. However, that does bring up the interesting point of the Labs. These were mentioned earlier, and it would be fun to see players just get the hopelessly damaged sections of their body replaced. You did well on that mission? Great, let's replace that shattered arm of yours with a cybernetic monstrosity that's been infected by saigai contaminated insects and shoots fucking plasma wasps! You brought as back a tin can and a moaning head? Well, we know you got your legs chewed off by a mutant chinchilla, so we krazy glued you to a wheelchair. Go out and gather more samples if you want some real legs!
So, now that we've got that idea in place, let's look at genemods again.
We can do four main slots of legs,arms, torso, head. Or we can increase that to six by doing left and right arms independently. Either way, I think we should separate it into minor/major and malicious mods. Minor and major mods go in the normal slots, but malicious mods simply accrete without an easy means of removal. (No cutting an arm off to get rid of a bad one!) Malicious mods should be added under certain circumstances, such as when the player is killed (as a rez cost), or perhaps even during certain operations to change out damaged limbs (tissue rejection).
So, let's say that we give the players two minor genemods, one major, and one malicious. For a sort of Corvian build, we might go with...
Torso: Stunted Wings //Limited Flight
Left Leg: Raking Talons //Extra melee with bonus damage based on fall distance
Head: Murder Call // Can scream to call in/disgorge multiple small ravens to damage and distract targets.
Malicious: Brittle Bones //All malicious effects directed at limbs are treated as having one more chance than usual.
Now, a few things become apparent. Putting slots into your torso and head likely means you don't intend to be switching those out any time soon. We also need some kind of numbered stat system that determines health/melee damage/ranged accuracy/etc. After playing Endless Legend with Dwarmin, I'm a bit fond of how they handle dice rolls, so let's run something like this...
Each stat has a rating from 0 to V. The actual effect of those is described on the stat, but will usually amount to an extra die rolled. Probably give players 5 points to spend at start.
Strength: Adds 1d6 to melee checks
Vitality: Adds 6 to total health
Dexterity: Adds 1d6 to ranged checks
Agility: Adds 1d6 to evasion checks
Tenacity: Adds 1d6 on luck checks
Rolls all get a base of 1d20. The D20 does not explode. The D6s all do.
And... This is going much longer than I expected. You can sort of see the problem.
Give it a try. Think of it as a game eternally in alpha development hell.
This is genius. Absolute genius as a theme for a game. The terribly thought out stat and damage systems are the DM's mistakes, they're bloody features.
Anywho, I've got to go. Who knows, maybe I'll come back with a more complete system.
*Side note: Bad Romaji would be a good Gaga parody