I feel as if Sailor Saturn was the very first Sailor Senshi to reside in the Milky Way Galaxy.
I have a head cannon where she was the first to greet Queen Serenity, and even warn her of her powers which is why Queen Serenity gifted Uranus, Neptune and Pluto with talismans that warn of her awakening with the intent to use her most deadly power.
If this were to be true, than every single senshi within the Solar System had their costumes based off of Sailor Saturns design, as I believe someone at some point had to create and design the uniforms for senshi to wear like a ceremonial cloth.
The individual planet will all have their own ceremonial costume for their planets, but certain Solar Systems like that of the planet Kinmoku star system have matching uniforms because when Alliances are created, their uniforms showcase they no longer protect one planet individually but each Senshi protects the Galaxia like a military in uniform fashion, literally haha!
I feel as if Queen Serenity knew a LOT of things were going to happen, she certainly prepared for as much including creating a computer program to contain her soul in the afterlife to eet her daughter. How'd she know such a thing would be needed?
I mentioned on another thread, that certain jewels on Tiaras are like power stones. In the original fukus, they were all very similar but some had added decoration.
Sailor Mercury and Pluto lack shoulder pads.
Sailor Mercury's earrings create a holographic visor screen.
Sailor Mars has a decorative "power gem" on the V of her fuku.
Sailor Venus transforms with a whip. (Let's not forget, her Sailor V costume. We'll get to that shortly.)
Sailor Jupiter has a rose on her belt, and her Tiara extends to a Lightning Rod.
The variations go on. I feel as though they were designed with the general details in mind but were alerted slightly by their personal specifications.
Some didn't want the shoulder pads. Everyone got to pick out their own shoes. Etc.
How does this work with their upgraded, even more similar uniform styles?
Well if Queen Serenity based all the senshi's designs to go along with the original Solar System senshi, Sailor Saturn, I feel as if it went down like this.
They somehow knew of the outcome in the future, at least up to the Super uniform. Saturn wanted to live WITH the rest of the Senshi. Something she learned to believe she could do when she was reborn.
In this theory, She WANTED uniformity, to be with the others, probably just as much if not more than even Pluto, Uranus and Neptune who wanted to be with the inner court too. But in the Silver Millennium, her life had created a different line of thinking but she awaited the day she could join. When it comes time and she transforms, she still has this line of thinking and old uniform, but it changed when Super Sailor Moon saves her from the fate of her strongest power.
And prepared for this moment, the Soul of her old self gives Hotaru, when she is ready to receive it... in the bath tub lol... her brand new uniform hidden inside her star seed the Heart Shaped Saturn Crystal, along with the others crystals (They were being upgraded, which is why they could not transform until the original Sailor Saturn was ready to give it to them)
I imagine Princess Serenity never got to design her own uniform because Queen Serenity knew she wasn't ready and that she was going to die anyway and that went all to her mother to design, with varying changed brought on by emotional power that can manifest, transform/change and create.
So the bases of the uniform is there, but without the actual transformation item, the Sailor Crystal compensates the original design that can't be accessed into a similar design that's weaker(Sailor V) or stronger (Eternal Sailor Moon).
Sailor V's design is an example of this emotional change since both Minako and Artemis were missing chunks of their memories, like Luna and the rest of them. Her emotions manifested a crescent moon because as we see in Codename Sailor V she remembers the moon princess as her most important person, or her memory was manipulated specifically in a way to emotionally bring about that change for the sake of being a decoy by Queen Serenity. The rest of her uniform might have been compensated for the fact that she associated her original senshi uniform of that with her own school uniform.
I figured that Sailor Moons designs were meant to showcase the difference as a new leader because originally her outfit had just about as many different variations as any other, but as she grew in power and in status and was seen as the leader, her uniform showcases like that much like you have different military members dressing in different badges according to rank as well.
I find this theory fun because I found it somewhere else, and they said it because they designed original uniforms for every senshi's planet different of the pleated skirt and white leotard because with this theory, they all have a ceremonial senshi uniform, and then they have their silver millennium uniforms(Anime only, the greek styled light colorized greek like uniforms.) and then all uniforms after that.)