Okay, you opened the floodgates on opinions:

First up, there are a lot of interpretations of fashion design designs for Tolkien that I just plain hate. I have very specific ideas, and if the clothing designs and choices are too generic, don’t have overall consistency, break world-building conventions, go too far astray into RPG or Victorinisms territory, or are just boring…I throw my private judgmental bitch-fit that I do for all costuming for fantasy works. I accept that I have very distinct ideas for many groups and time periods in Arda and until I get off my ass and draw/share them, I can’t be judging. So, you want the master-post list of the fashion headcanons? I’ll try to list the majority of what I’ve come up with, roughly chronologically:

Cuiviénen. GO LOOK AT PREHISTORIC CLOTHING. Examples and reconstructions do exist. Okay I don’t think we have a tech and civilization level beyond low Stone Age but they probably have weaving of textiles by then. But for the first couple years? No. But does that mean no intricate hide garments covered in hundreds of beads made of shell and bone? Give me that. Give me that instead of white robes. 

I could go on and on about the fashion of Cuiviénen alone, I know I have. But for transitions- linen is discovered around the time of Oromë’s arrival. Layering with proto-linen garments under the tube skirt/dresses and leather leggings is a thing. Egyptian-style linen garments is going to be the inspiration for some Avari and/or Vanyar. But this is the Wastonian answer for linen shifts and those lovely white undershirts.

Sindarin fashion evolves into this blend of Korean hanbok and Italian Renaissance. I’m sorry, but the fashion of the early Renaissance is infinitely prettier than any medieval look. The same way Victorian fashion, be it the bustle skirts or the early hoop skirts, are actually really ugly and Empire/Regency fashion is prettier. Am I biased towards empire-waistline garments? Hell yes. Sindar in Beleriand wear a lot of silk. It is the primary textile. Armor is padded layers of silk (effective!). Queen Melian showed them how to harvest the silk from silk worms. The mulberry trees and secrets of silk production are mostly lost when Beleriand is destroyed. Men’s fashion is pants or hose over a few layers of tunic and a jacket, then over robes or a mantle. Bright colors and very striking patterns- outside it might be dark, but inside homes the Sindar are going to have illumination- why the heck does anyone think inside Menegroth it was twilight? Colors like bright red are going to show up as camouflage-worthy gray when hunting outside – it’s reflective shiny stuff and pure blacks to avoid- there is no worries about hunting in daytime. Beleg has bright red shoes because he’s too lazy to switch over okay and he spend years breaking them in to be really comfy. Patterns to break the silhouette. Older styles have the skirt and short jacket pieces as separate (sort of like the chima and jeogori) whereas later styles sew the vest to the skirt and the sleeves are the detachable part. Again layers of white undergarments. Lúthien was wearing an old style blue dress and removed her over-vest/jacket to have those bare arms as described in the Lay.

The slashing trend was not because of the Swiss Guard this time but survivors returning from fighting Morgoth that first time.

Noldor are Tudor. They love their towers and castles, there are gable hoods to reflect that. If they can invent cubic zirconia (jewelsmiths~) in Valinor, they are inventing those bright Victorian synthetic dyes- and showing an equally Victorian lack of refined taste in using said new colors. Some elven asshole probably invents neon bright dyes and the Noldor decide that is the fucking best and color-palette wise the sons of Fëanor look like the unholy manifestation of the 1980s. Rainbow everything. Tacky elves. Shiny spangles all over everything. Most detail work is possements and embroidery and jewelry that is attached separately to garments and can be changed out or added to a new piece. Sindarin tapestry patterns versus embroidered fabrics with heavy use of metallic threads. (This is why the looks aren’t perfect matches with the inspirations). They take fashion ideas and run with it and add structures and exaggerations, so that’s why the hint of the linen undershirt peeking up at the collar turns into those huge starched ruffs needing hundreds of pins that also do a great job of creating this halo/negative space around a person’s head and hair as to highlight it. I’m sparing the codpiece, but frankly I could be talked into it.

Vanyar are simpler. Falmari more sensible fabric and fabric dyes.

Noldor stuck in Beleriand are subdued down from that Jacobean to Tudor look, because historical fashion inspirations in reverse. The look in Nargothrond is Cranach gown/Northern Renaissance. By the end of the First Age it’s those Burgundian gowns of the 1400s. There wasn’t enough fabric to waste for houppelandes.

Edain fashion turn to tradition European folk designs, especially Central Europe, look at how detailed the embroidery is, but also remember that this period is basically early bronze age to 500-1100s Europe with the dissolution of civilizations, look, Tolkien, call it the Dark Ages. The alternative to Tudor!Elves i accept as valid, by the way, is Roman!elves.

Bëorians have Tartan. Hadorim are free to Viking up the heck the way the Rohirrim were. Haladim more subdued than both, but some lovely bold black and white patterned fabrics- hounds-tooth, checked, stripes. Trading with Doriath will do that. Their sleeves, unlike the other Edain, tend to be dolman –  the sleeve is not cut separate and sewn on. It’s a weird detail.

All three groups have distinct hairstyles. Beards are considered normal and manly.

Bòr have a few differences from other Easterling groups- outsiders can’t tell the difference but they can. They don’t wear beards, hair in braids. Men have two braids, women wear multiple braids, elaborate updos, there is a look sort of like the Vestal Virgin’s sini crenes where the number of braids and the complexity meant that a woman wearing her hair like it was religiously important and/or a chieftain’s wife. Said looks would need a hairdresser to help- the Easterlings have thralls, it’s a sign of wealth. Women wear loose trousers over long jackets. Boots with an upturned toe (Mongolian boot) is a sign of a tribesman related to Ulfang, not Bòr. 

The returning Noldor and Vanyar from the War of Wrath bring back Beleriandic styles and especially a craze for everything and anything Edain. This is Neo-Classical Empire/Regency stuff. High waist, tubular skirts. Designs are simplified with an emphasis on severity before relaxing back into frou-frou craziness. Short hair a la Lúthien from the Lay of Leithian is Most Fashionable. Fake beards or at least fake sideburns. Imitation Edain patterns.

On Númenor the silhouette is baggy trousers and short tight vests for men, lots of wool and sheepskin. Women have dresses that are close to those Minoan frescoes in their look. Hourglass, padded hips, a return to that early Noldor look. White undershirts with embroidered cuffs and neck/chests on everyone. For a while the trend for men’s shirts are to have absolutely huge bell-sleeved shirts that completely hide the hands- sign of a nobleman. Tight laced vests. For women though the skirts are this huge many-tiered (traditionally white in some regions, others very colorful) skirts with small aprons. Take the Minoan Snake Goddess and combine her with Mexican folklorico dresses (especially Veracruz). Loose-fitting plain white clothing is sailor garb. No beards. Short hair. Women have curls piled high atop their heads. Lots of cute sunhats worn by everyone.

This jacket/vest over a skirt combo survives in Umbar.

Chainmail is the default armor. Full plate-mail is War of Wrath Amaynar troops. Plate armor seen elsewhere is the segmented plate of the Roman Empire or Qin Dynasty.

Oh, yeah. Rococo Ainur. That. :p