Many of the mutation categories are animal-related. Could we have more non-animal categories such as Alpha or Medical? Could any of the following be categories?
(I’m just dropping words here hoping it’ll spark some inspiration)
Chemical
Subatomic
Quantum
Inorganic
Industrial
Mineral
Elemental
Psychological/Cognition
Cerebral
Immaterial
Star/Astronomy/Celestial
Sound
Hedonism/Euphoria
Sensory
Newtonian
Microbial
Temporal
Electromagnetic
Mineral/Crystal and Quantum stand out as fun ideas.
If you want to stick with strictly biological mutations, Coral or Pangolin could be close approximations for a very rocky, durable mutation path. Ooh, if you went Pangolin you could have a “ball” mutation that turns you into a 1 tile vehicle that can ram enemies 
As far as natural armor, the vanilla mutations are along the lines of fur/scales/snail shell/natural regeneration. A path dedicated to being heavily armored would be slightly different, especially if said armor is quite spiky. Downsides could include slowness, sleepiness, or becoming an electrovore (eats batteries instead of food for nutrition). As is this sounds too similar to Plant so it would need some work.
Quantum sounds good, seeing as the base game already has Alpha, Medical, and Elf mutations along with lots of teleporter/portal technology. Would likely include teleporting/phasing perks in exchange for teleportisis, mood instability, or perhaps stat instability?
Celestial. Star-like properties at cellular level, usually at the cost of metabolism.
Trait-like
Illumination (minimal) - Your body can produce light at will, at the expense of metabolism. This provides the ability to produce minimal illumination, barely enough for reading in the dark.
Anti-Glare Vision - No amount of light can blind you anymore. You’re not going to need sunglasses or welding glasses anymore. Unless for fashion purposes.
Normal
Illumination (substantial) - Your body can produce light at will, at the expense of metabolism. This provides substantial lighting, enough to make you a beacon to monsters, or, for example, comfortable navigation in the dark.
Increased Tissue Density - Your tissue composition has become substantially more dense and solid. Your body weighs more and you gain considerable resistance against cutting damage, but you also require more energy to move, increasing metabolism and stamina consumption while moving. On the upside, you can still receive injections from standard steel needles.
Increased Bone Density - Your bones cannot break anymore, not by any normal means. But you become heavier, and require more sustenance. Your body also needs more calcium from diet.
Dulled State - All your senses with the exception of vision are dulled. You are more resistant to pain but also more resistant to negative and positive morale modifiers.
Solar Flare - You can expel hot plasma from your fingertips at the cost of hydration, causing massive burn damage to anything the plasma hits. Activating this power will also automatically destroy any handwear you might have equipped, so try not to wear gloves or watches, or at least remove them first.
Universal Matter Absorption - Your body can absorb 500g of any material per day to recover hunger, equivalent of a moderate meal. The absorption takes place through skin so you don’t have to chew rocks or anything - just hold a rock (or whatever) in your hand and activate this ability.
Fusion Metabolism - A small fraction of your cells have been transformed and they now employ fusion reaction to produce energy. This greatly increases your resistance against cold and grants you extra strength, but your metabolism is slightly faster.
Quasar - Occasionally random parts of your body produce an intense flash of light. The more covered your body is, the less likely this is to show on the outside. Partial body coverage reduces the flash visibility as well.
Post-Threshold
Illumination (extreme) - Your body can produce light at will, at the expense of metabolism. This ability is enough to temporarily blind monsters and NPCs at the expense of substantial metabolism and also stamina. Use sparingly.
Black Hole Stress - The more in pain you are, the more violently loose objects start to move around you, causing havoc, but at least they never hit you directly.
Black Hole Darkness - Your body absorbs light, making your immediate surroundings permanently dark. Darkness follows you wherever you go. This is exceptionally disturbing to NPCs.
Black Hole Disintegration - You can pulverize any creature, item or tile with your touch. This action consumes a substantial amount of your focus, metabolism, thirst, fatigue and tiredness. Use sparingly.
Omnivision - Your eyes can see almost any wavelength. You can perceive radiowaves, the ultraviolet, microwaves, gravitational waves, the infrared… you name it, you see it. Nothing can hide from your eyes. You automatically map several levels beneath (or above) your feet, as well as can see any monsters there. On the downside, closing your eyes is basically pointless now as you can see through them.
Neutralized Quasar - This is the next step of the Quasar mutation, nullifying its effect. No more waking up in the middle of the night just because your face flashed like a camera.
I’ve always wanted Rabbit and Porcupine/Hedgehog paths. The first to add a ‘small’ mutation to counteract Bovine and Ursine ‘large’ mutations, and the second to give the Spines mutation a home.
For Hedonism/Euphoria
I was thinking of a combo power storage/generator CBM mechanic, wherein you build up [Power] under very painful circumstances (ex: getting near raging fires). In exchange, you would be able to enter an altered state of benefit (haven’t thought deeply on the matter).
I didn’t think of that, but you’re right that we need “small” mutations to balance out the “large” ones. It could be added to a few of the existing mutations, such as Bird (would fit with their high dodge and low hp).
Other than +dodge and lower hp, I can’t think of many uses for a small mutation. I suppose you could make it part of a path for burrowing/not setting off mines/quiet(er) step/not sinking in mud and so forth.
Since the ranged accuracy system was reworked, it is possible to have different sizes for ranged weapon accuracy. A small creature could be harder to hit with ranged weapons, even if barely more dodgy in melee.
My understanding of mutagen is that it has to mostly work with biological materials, so things like mineral don’t make much sense, at least to the extent that they differ from some sort of crustacean/mollusk type mutagen. Also, the non directly single animal class mutagens (alpha, medical, elf-a, etc) were supposed to have been just developed as the cataclysm was kicking off, so it seems like some of the more advanced/tricky mutagens would have to be extremely rare (and probably a low variety).
It might be interesting for gameplay purposes if, let’s say 20 “advanced” mutagens were created, that the game might randomly pick 5 of them to spawn in any given game. It could also be possible to gather NPC mutagen scientists who can finish research on the other 15 over time as an end game goal.
The mutagen lore isn’t consistent.
It tries to combine “mutagen is a thing that makes organisms mutate” and “mutagen is a thing that makes blob infection cause mutations”.
In the latter case, it is totally fine for it to cause alien mutations that don’t exist in animals or plants.
I do hope it’s the latter, because I’d really like some “psychic” and otherwise netherworldly mutation categories.
The Mineral one stood out to me for some reason, and then I realized: Silicon. A Silicon mutation tree inspired by silicon-based life. Silicon-based life technically isn’t feasible, but who cares.
The Silicon tree could basically make your character nearly unkillable like the Medical tree, but you would be slower, get Glass Jaw, and have more than a few biological “complications” due to conflicts between carbon and silicon biochemistry.
Particularly interesting mutations could include a silicon shell or silicon armor akin to chitin armor, silicon spines, electrical resistance/immunity, and electrical discharge. Maybe a boost to intelligence because electrical signals in the brain or some other pseudo-science.
We don’t even know for sure if silicon-based life is viable, and even if yes, I do not imagine it being in any way compatible with carbon-based life. Not to mention everything we eat is carbon… it would be a huge mess.
We don’t know for sure if other dimensions exist, much less that travel between them is possible, but I don’t see any complaints about that. It’s fiction, anything from hard sci-fi to magic to the kind of stuff you made up on the elementary school playground is fair game.
You know what else is a huge mess? Sprouting limbs spontaneously by drinking green goo. Or suddenly transitioning from warm to cold blooded. Or becoming an omnicellular organism. Or your blood turning into freaking acid.
Sorry if I come off as aggressive, but the last comment I expected in a zombie/interdimensional Lovecraft apocalypse is “It’s unrealistic.”
Edit: I feel like my response was disproportionate. Must be the mutagen induced mood swings.
Yeah, but this is an entirely different class of “likely infeasible”, and probably could as well be done with the more fitting into the existing lore mutation category, based off the horrors popping up or whatever.
I think the main thing that gets me about super crazy mutagens is the timeline. Even the most cutting edge mutagens currently in the game were developed as the cataclysm was happening. For the scientists to also have weird alternate non-biological mutagens at the same time seems a little advanced for the timeline. It would be interesting for me if developing these new and exotic mutagens were something the player could somehow work towards as an end game goal though.
Well, the labs do have nether monsters every now and then, I think it would be more or less reasonable. And fun, either way.
Cockatrice, a postthreshold trait for chimera.
Fleshy monsters that hit you in melee turn into large boulder, small boulder, or rock, based on their size. Zombies can imprison an otherwise unkillable chimera within rock walls.