A Fith-Fath (fee-faw) is a term from Celtic Mythology meaning a "Transformation Chase." Transformation can mean many things, and is rather complex as a topic...
Ah, confound it. I just wanted to try to put a certain shapeshifting cynic into a rational world and got carried away as usual.

It is a complex world, where there is no obvious blue space magic or blessings that are known to work...but neither is there a set science, and several mysterious things happen across the world. Quilled "feathered" cats, man-sized birds, child-sized holes that pop out of the ground from nowhere, and the ability of a humanoid race to change appearance through what they are exposed to.
All the while, technology marches on...but the gunpowder won't combust...

Characteristics:
A Low Fantasy/Hard Science Fiction world (Different Planet than Earth)
-Based on a mix of cultures around the world in the time before Charlemagne
-And Early mythologies
-Where biology is manipulated to allow for Transformation and mythical creatures (things like changing hair growth, or diseases that alter/damage bone/muscle structure...so a metaphor for medical/mental issues and Sensory Dysfunction...and on the other side, a Horse-like amphibian to get something like a Water Horse, or a panther-like Crocodile)
And technology of the time is more prevalent (Not castles and knights...but Archimedes's Diving Bell and a City build as a giant early Observatory...)
Slower Pace - more focus on character interaction...and we don't have cars and planes and motorboats...
Supposed to be a large enough world to let other people tell their own stories within - like Faerun, or Warhammer...only there can be a range. Some societies are crumbling, and to the locals the world may be under a seemingly apocalyptic plague...or a noble may be poisoned and a mystery ensues...or a scientist's quest to develop the fundamentals of science and technology...or a religious experience...or encountering a fantastical creature...or romance on a year-long trade journey...or political intrigue...or piracy and dangers/marvels at sea...or an aesop or origin story told...or even some degree of Cosmic Horror when bones begin to rise...

Goliatha
Onile
Gwion ab'Arlnett
Sand ab'Foel
Wadjet-ba
Arroqtshehip (Errol Flint)
(Character #1)
(Character #2)
(Character #3)
(Character #4)
(Character #5)
(Character #6)

Kadia & Rurik (Totally not a schizoid character with a messiah complex and her all-purpose cyber-dog)
Morwyn ap'Cludd
(Again, need names for several characters...will get to them eventually)

Things I've begun to look into:
Sulfur and Selenium - alias, how to avoid Black Powder and accidentally mess up DNA royally
Hair, Fur, Quills, Nails, and Feathers
How different Embryos Grow
Hormones and Puberty
Transformation as a Metaphor
Limits on Limbs (Or, how a Western Dragon would mean we should have 4 arms)

Military
Agricultural
Production
Trade
Knowledge

Superstructure

a) Worldview (“All are represented by sacred Math” or “Mankind has Sinned before God”)
b) Mythology (Origin story? Values through…something? Methods of worship?)
c) Knowledge (Mathematics? Impractical Steam Power? Astronomy? Military?)
d) Values (Individualism would be a Western value…perhaps honor, or memory, or even a certain physical form, etc.)

Social Structure

a) Language (Symbols, Body Language, Phonetics, Dialects)
b) Distribution of Resources (Trade and Tax)
c) Rules (Laws and Codes - how they are enforced)
d) Class/Caste (In Caesar’s Celts - the Druids, the Equestrians, and the Civilians)
e) Subgroups (Political Parties, Exiles, Subcultures, Good mythical example being the Fair Folk)
f) Roles and Relationships (Marriage, Friendships, Gender, etc)

Infrastructure

a) Subsistence Strategy (How a culture survives – Nomadic? Animal herding? Crop Rotation?)
b) Production Technology (Smelting? Cranes? Plows?)
c) Overlay of cultural centers (Cities, Roads, Nomadic Paths)

Environment

a) Resources available (Salt? Wine? Wood? Ore?)
b) Threats and Dangers (Lions and Tornadoes and Faults, oh my!)
c) Natural Events (Seasons?)
d) Locals (the biome, non-threatening animals, other cultures)

The Midewin peoples are also known as the Weaving people. They use a mix of natural fibers tied with many layers of animal sinew and wild silk to create the thick three ropes used in their suspension bridges. Their ceremonial clothes* are woven by hand with feathers around the shoulders. Of their written language, it relies on weaving White Birch and Black Maple bark together. Counting uses a knot method similar to the Japanese Multiplication Method through Lines. They use cross-hatched nets and hooked spears to catch fish, and hunt deer with longbows.
Though largely reliant the movement of various brackish and freshwater fish, they have cut away patches of land for pottery clay and for the creation of terrace-farms. On these farms, Longhouses, raised from the ground on Red Cedar wood, provide shelter for extended families.
Around a select few caverns they have constructed ashlar stone walls and buildings – forts and a temple – as an urban meeting site. It is presumed these locations are used in the winter months, using caves for shelter. During these times, compacted snow is used to preserve meat. Despite a lack of soft stone, the temples are a beautiful with copper** and gemstone murals. Wooden cubbies hold records of their mythology and transactions. It is during this season that children are taught mathematics and their history.
During the spring and summer the Midewin tend to construct quick shelters of wood and straw as they hunt and gather. Certain apprentices are taught skills ranging from herbal medicine and crafting to weather-watching and basic astrology over time as they stay and care for the temples.


*Work-wear, along with most pouches and moccasins, is made of wax and leather.
**Copper is sifted from the riverbanks running from the higher mountains through gravity, and then smelted and reforged into copper plates. This runs the risk of encountering a more feline crocodile similar to a small Kaprosuchus.