See the new SSCS Generator on my website here!

It's nicer, easier to read, and let's you fill it out for your specific character!

I have several different shapeshifting characters, so I decided to make a categorization system for classifying different types of shapeshifters.
I'd really like to see this adopted by others, I think it's a concise way to describe the capabilities of a particular shapeshifter.

Pecacheu's Shape-Shifter Categorization System

Transition Styles

C = COREFORM, Transformative goo surrounding a solid core/body.
F = FREEFORM, A shapeshifter with full or partial transformation freedom.
M = MULTIFORM, A creature that can change between multiple static forms.

The combination CM is possible, a shapeshifter with a solid core, but that can also change into several different states. The creature may have a semi-goo and a non-goo form, for example.
Also possible is FM, a shapeshifter with freeform transformation but with several natural forms.

Shape-Shifter Types

1. Psychic
An instant, non-physical shapeshifting method which manipulates the mental perception of viewers. This either doesn't work through video cameras, or only works on unedited footage. Typically unreliable and easily defeated, however has the advantage of seeming solid due to manipulation of the observer's sense of touch.

2. Illusion
Non-physical shapeshifting via optical illusion / light manipulation. This always works through regular visible-light cameras, but not necessarily with other image sensor types. Illusion is the only type of shapeshifting that isn't solid to the touch.

3. Assimilation
Your typical movie shapeshifting supervillan, these can only transform into other living creatures, often having access to their memories. It either uses a remote connection of some kind, or consumption of the target. It's not really a type of shape-shifting easily explained with science, yet it's the one used most frequently in the Sci-Fi genre. They also sometimes don't have a default form that they're forced back into (ex. when sleeping), like most other shapeshifters do.

4. Object
Transformation into specific objects or living creatures, often requiring previous physical contact with the object in question. Object shapeshifting doesn't allow for freeform transformation, and only allows for limited partial transformation, however it's the most accurate type of shapeshifting, and can pass virtually any test. Also difficult to explain scientifically; the usual explanation is "because ghosts."

5. Goo ("Goofur")
A shapeshifting method in which the entire creature is made of a semi-liquid substance, allowing for the most transformative freedom, but at the cost of being a constant inconvenience in general day-to-day living. Goo type shapeshifters are not capable of making themselves 100% solid, and are often limited to one or several colors, so it's a little less than subtle, however they're virtually indestructible, self-repairing damage from bullets or knives. The Encapsulation ability is pretty much a given. If made of a synthetic polymer, a Goo shapeshifter may not even need to breathe, and is impervious to disease or poison. The Color Shifting ability is used to denote if the shapeshifter may change it's colors.

6. Adaptive Goo ("Liquid-shifter")
The capabilities of a goo shapeshifter, but with the ability to make themselves completely solid (sometimes even fur-covered), hiding their shapeshifting abilities. Not all Adaptive Goo shapeshifters can become solid when not in their default form, but they can often make themselves partially solid and partially goo. Liquid-shifters who grow fur are typically more vulnerable when in their furred state. The Fur Shifting ability is used to denote if the shapeshifter can grow/replicate a fur texture.

7. Material
The best balance of capability and transformation accuracy, Material shapeshifting is usually explained via cells that can dynamically change their properties to replicate the texture and appearance of a variety of materials, liquid or solid. Allows for the same transformative freedom as Goo shapeshifting, but with the ability to accurately take the form of other creatures and objects, including their color. Replication is certainly not perfect, especially of non-organic materials or liquids, and will fail more invasive tests, but will look correct under most lighting and will fool some basic, non-contact tests (palm scanners, thermal cameras, etc.)

8. Virus
Virus furs function internally in a similar way to Material shapeshifters, but generally have limited transformation capabilities similar to Goo or Adaptive Goo types. They are infectious shapeshifters who spread themselves on contact with other living creatures. They are typically unable to split/duplicate by themselves (without a second host), technically making them incompatible with the Duplication ability. The Fur Shifting ability is rare with Virus furs, but useful for disguising as a regular animal. Virus types are also the only type that always has the Merging ability, after all, what's the point in a virus that can't spread?

9. Adaptation ("Shape-Changing")
Creatures (or appendages) that can change their exterior shape to a limited degree, but are not full shapeshifters. Typically limited in possible geometric shapes, such as maximum curvature of convex geometry, maximum protrusion distance, maximum sharpness of acute angles, etc. Usually works via a structure of muscles/cartilage/etc. underneath a flexible membrane. Not compatible with any sub-abilities, as it's not really a shapeshifter type.

Sub-Abilities

a. Color Shifting
The simplest sub-ability, allows the shapeshifter to change their color(s). Only applies to Goo, Adaptive Goo, or Virus types; required for Assimilation type; implied for all other types.

b. Fur Shifting
The ability to actually grow fur (True Fur Shifting), or to accurately replicate the texture and appearance of it (False Fur Shifting). Only applies to Adaptive Goo or Virus types; incompatible with Goo type; implied for all other types.

c. Encapsulation
Ability to engulf and encapsulate other creatures/objects, either for regular digestion (NOT absorption), or simply to hold them captive. Not compatible with non-physical types; implied for Goo, Adaptive Goo, Material, and Virus types.

d. Absorption aka Merging
Ability to absorb, merge with, or merge into other creatures, gaining their abilities and form, sometimes giving them partial control over your own. In combination with Duplication or Shared Shifting abilities, this allows for prey Assimilation as well. Usually temporary. Not compatible with non-physical or Assimilation types; implied for Virus type.

e. Object Fusionaka Object Merging
Allows shapeshifter not only to encapsulate, but actually fuse into existing objects, potentially taking control of them. Often slower for large objects. Not compatible with non-physical, Assimilation, or Virus types.

f. Phase Through
An extension of Object Fusion, allows the shapeshifter to phase through an object by merging into it, then back out on the other side. Requires Object Fusion. Not compatible with non-physical, Assimilation, or Virus types.

g. Duplication
Ability to split into multiple entities or pieces, typically without separate personalities. In most cases, these pieces have the same memories but are isolated until they "sync up" by rejoining. If the shapeshifter is able to telepathically control their pieces while separate, it is referred to as Synchronous Duplication. Only compatible with Goo, Adaptive Goo, and Material types.

h. Shared Shifting
Ability to shapeshift others on contact. For Virus furs, it may be possible to fight with a strong enough immune system. For other types, there is usually a time limit before the target returns to normal. As with Duplication, if the shapeshifter can communicate with itself remotely, it is Synchronous Shared Shifting. Potentially compatible with all types; Synchronous not compatible with non-physical or Object types; required for Virus type.

i. Camouflage
Constant, adaptive color changing to make self invisible. The shapeshifter may even be able to reflect non-visible light to make self invisible even in infrared. Requires Color Shifting. Not compatible with Object type.

Limitations

Because shapeshifters are so overpowered, they often have limitations placed on them, to keep them from simply never having problems. These limitations typically include material limits, energy limits, and speed limits.

Material limits mean that your form can only take up so much volume until it can't become any larger. This volume is determined by the current amount of material in your body, and your maximum density. There's also a minimum density limitation, meaning you can only shrink so small before you're limited as well. Usually, gaining material is done through eating, and occurs over the course of several hours, while losing material happens naturally over time, until you're back to your default. This means your much less likely to get fat in your default form as a shapeshifter, though it is possible.

Energy and speed limitations are much simpler. Simply imposing minimum energy consumption levels, or a maximum speed, on your shapeshifting. Speed limitations obviously don't apply to the INSTANT transition. Energy usage usually changes depending on the difference between the old and new forms, and based on usage of any sub-abilities.

Example: Zithon the Fox

Zithon - FREEFORM Virus - Color Shifting, True Fur Shifting, Synchronous Shared Shifting

As an example, let's use Zithon, our resident happy-go-lucky shapeshifting Fox! Zithon is a Virus type shapeshifter with INSTANT transition and Color Shifting and Fur Shifting abilities. Being a Virus type, the Encapsulation, Merging, and Shared Shifting abilities are automatically implied. This leaves Object Merging, Phase Through, and Duplication, which are incompatible, and Camouflage, which he does not possess.

Most shapeshifters also have a default form. Zithon's is a male Red Fox cub (Specifically, he's from a den in the Siberian Tundra). Shapeshifters are typically forced back into their default forms when injured, sleeping, or sick. Also, be warned that sleep shape-shifting is a thing.

Zithon has both material and energy limits, making him less unreasonably OP, as well as more "compliant" with the actual laws of Physics. His material limits go from 1/2 to 5x natural density. For example, if Zithon starts at natural density, he can only split himself once into 2 full-sized copies before hitting his minimum limit. However he can continue to split himself into smaller copies, as the limit is only for the density of each copy. (Edit: NVM Zithon doesn't have Duplication ability so he can't split but you get the point...)