Setting Design I and the first race

The setting I run is called "the sphere", unlike many space opera settings with hundreds of planets and hundreds of different alien species. I don't do this because I hate aliens or am some boring stick in the mud, rather it's a question of avoiding generic tropes that so many movies fall into, negating some of the "damage" that is introduced by superscience such as FTL drives and similar and to actually make something different.
Instead of Star wars, 40k and other very popular sci-fi franchises that is popular in my circle of players I want to look to some classic sci-fi stories.

Illustration by Michael Whelan, I like the scene and I liked foundation. Now to make a good homage.

For this I looked first to Isaac Asimov's foundation series, a classic from the 50s.
I loved the fall of Rome narrative that was one of the keys to his story, however I found the notion that humans had colonized the entire galaxy and not seen any signs of life other than human silly. Even with the drake equation and the sheer statistics of the entire galaxy and only Sol 3 developing life. I took notes, I need a variation so it's not just humans.

Second I looked at Larry Niven.
Here I loved his concept of belters, the concept of mankind dividing and part of it living in space. I took notes on this.

Third was Dune.
I loved the idea of Psionics and how FTL was a central central point, again the notion of a decaying empire so far away from our time that we lose all connections to history. Notes were taken.

Looking through my notes I found the concept of a divided humanity interesting. It could provide me some of the things I need for my setting. Wanting limited worlds and a divided humanity? Pantropy! I'd have to limit the abilities of terraforming. Interesting, most settings assume oxygen nitrogen atmospheres are totally normal. Maybe worlds that can be terraformed are rare and terraforming is expensive. I like that, thinking about it even terraforming Mars would take generations and our modern politicians have difficulties thinking beyond the next election cycle. Look at modern climate problems: Politicians cant leverage the short term profits of modern production against long term environmental damage and the costs it will bring.
Now I am cooking with gasoline.

So I want biomodified humans, a limited amount of worlds and harsh environments. Right. This also deals with the problem of no variation: The biomodified panhumans replace aliens in my setting! One problem of the list.
Now I also need to deal with superscience FTL drives. This will decide the scope of my setting and will be quite a trouble... Hmm. I needed to refine the scope. After debating it with one of my buddies, TheCommaBandit I settled on a 50 light year diameter globular cluster.
How do I make this work?
Looking at the fall of Rome scenario from Asimov I decided to roll with the fall of the empire for the whole lost tech part, the FTL and knowledge of it would be lost to the centuries with people only able to copy the drive. Perfect.
Limited worlds? Check.
Limited FTL? Check.

So I have a essentially medieval setting, I have my falling Rome. I need my barbarians, failing administration, ethnic division and stagnating economy.

Here it gets fun. I definitely need my panhumans now, they can provide my barbarians and ethnic tensions.
I decided on an ancient greek-ish elementals: Earth, water, fire and air. Here is my basis.

This would fit very well with not-so terraformed worlds. Now to make my race templates, do note that this is long after the initial process.

I decided to start with water. How do I make my water panhumans? What kind of "alien" do I want from these?

Illustration of the Asari from Mass Effect by Cynderloverforlife. As much as I like space babes there should definently be hair on their heads...

Initially I thought about the trope of the green skinned alien space babe. Most sci-fi settings have these: Star Wars has the Twilek, Mass Effect has the Asari and the Syreen from Star control. I want these, I need these. Especially since my group sees romances and sexual motivations very important for their gaming it's pretty much essential.
Asari here has a special place as they can roll with pretty much any character, male or female hmm... How to get this.

How do I make my human variation essentially bisexual, attractive girls? How do I deal with them not getting old? We don't ever see any wrinkly old Asari, Twileks or Syreen because it doesn't fit the trope... At the same time I need them to be underwater.
The Asari seem to be unaging but... Eh... I'm not sure I want that. There is another way of not getting to be old: Dying before they get to be wrinkly and saggy. Right, its better than the unaging and may even give them character.

So, I came up with the central empire designing humans for certain purposes not caring about ethics, might as make them an "evil" empire if they are gonna fall anyways. The biomodification is of low quality and they dont get to be very old good. Now to adapt them to underwater life and find a way for the biomodification to fail and not having them get very old. I started thinking of vital organs for some reason. Hmm... The reason we can't be underwater is our evolutionary choice of lungs over gills. Well, we are returning to the sea in with this race.
Lungs are replaced with faulty swimming bladder that fails catastrophically at some age, lets make it 50 years. Right, I mean then we can have some nice matur-ish space babes too.
If they were made to rapidly reproduce we'd want a lot more females with only a few males, we even get our reason for bisexual space babes there.
Add some slime to reduce friction and some skin between the fingers and toes, add a spoonful of eyes adapted for underwater and we are done.