Probably way to late for this, given product development cycles.
If I was emperor for a day, I’d be looking at this sort of model: Keep it Simple Stupid.
By this, there can be a lot of complexity built on later, but for the core product, just keep it basic. With templates to expand and make a bit more complex the core stuff, without the need to write pages and pages of rules.
Some random thoughts.
Races.
Use Sub-Races to get extra mileage.
Humans are a default pick for players in almost any campaign you visit, as every campaign has them. So assuming they will be included here, at the end of their section have some “Near” Human options too. (Borrowing from Star Wars for a tick, Chiss could be just blue skinned humans that lose the bonus feat and gain darkvision, and maybe a +2 to some skill. Cat-person, could be another that replaces feat with the ability to use a kitty litter tray really really well).
Elves. Already a space faring race in pathfinder, I’d think they would be appearing in this basic book too. So I assume you would have the basic elf stats, then maybe have some variants based on the whole Zulcan and Romulan ordeal.
Then perhaps detail the 4 people of the stars, each with an alternate version too (well lashunta, and Triaxians have alternates builtin), but maybe what is a Triaxian like born on a differnet planet. Are there Kasatha built for other environments, and Androids built for functions?
Classes
In the past, such as the d20 Star Wars, there were classes for pilot, tech guy, and a tonne of others. I felt they just ended up repeating stuff over and over again without really adding anything other than more and more rules. Given pathfinder has Archetypes, why not go down the path of using them as means to offer complexity in a simple system. Similar to the races above really. Offering basics, but options to flavour via templates.
Fighter,what setting could do without the face punching muscle? Since he is a must, maybe have some archetypes for more space bound fighters. Tactician is the first archetype that comes to mind, the captain kirk as it were. Trench Fighter (maybe reskined to “Soldier”) would be a good fit too for more shooty adventures the Technology Guide hints at.
Professional (rebadged from Rogue). This could be expanded upon heaps, the skill specialists as it were, from hackers to medics to ship staffers. For the base product, archetypes for the Sniper (I assume guns will feature prominently) and the Swashbuckler (its going to involve ships after all) would have to help flesh out the space feel. I’d also add a “Doctor” archetype (kind of like Drop Dead Studios’ “Medic”), playing on the idea that clerics on a spaceship would be rarer than a ships doctor, maybe something along the lines of “none can here you pray in space”.
Arcanist (yes that class, we aren’t trying to recreate pathfinder after all). Given the White Mage and Sword Bond give access to either a Healing Magic class and a Jedi like class through existing archetypes, to me it’s a more natural fit. With the way Arcanist memorises and casts spells, it gives the book a spellcaster without the basic wizard/cleric repeated so often. From memory, this class avoids Familiars, so that saves a bit of space in the book too.
No more than that really. There are a tonne of products on the market with classes galore. The above three core, with two archetypes each (well 3 for the Professional), gives TEN different actual classes to play with, and avoids excessive duplication from repeating the same stuff over and over. Speaking of saving space again, as there are no clerics to worry about, pages that would go into repeating their spells are saved for other content. So its win-win. Maybe to reduce concerns a bit, develop spells for Archaists that reskin existing spells (fireball becomes black bolt for example) and maybe have a couple of healing spells reskinned to White Mage only too.
By now we have saved some room with classes not getting overly repeated, and spells galore gone. Not to mention domains and a lot of class abilities too. These are available elsewhere, so they don’t need to get redone here. The races above could also give a stack of options without breaking a sweat (say access to feats that change something in a core class). So how does that space opera play out now?
Feats.
If this book is to be both self contained and allow backwards compatibility, there needs to be something for all classes to access that can get them space ready. Exotic Weapon Proficiency covers some solid ground. But what about vessels, information technology (even if its magic based) and whatever else makes the setting work?
Well we have Mounted Combat with its associated feats, so Vehicle Combat and Vessel Combat can be extended from that. Ride by Attack could be adapted to these, with a slightly different effect maybe. (Say vehicle version allows a RAM attack only, and Vessel allows a strafe attack only).
Beyond these, my preference would be to see feats that can stand alone as base feats, while adding flavour to a space setting. (Over and above the usual feats that is).
Skills
Please don’t go nuts with new skills. Driving, Piloting, Knowledge (Space) and Knowledge (whatever information technology is) should really be the only things needed (if even that) to run a space campaign. Heck Ride, Fly, Knowledge (Space) and Use Magic Device could be all that is needed, with the feats Vehicle Combat, and Vessel Combat providing a bonus to skill checks when used appropriately (i.e. no vehicle combat bonus for riding a horse, maybe go so far as a negative is applied unless the character has the right feat).
For instance
Vehicle Combat (Combat)
You are adept at guiding your Vehicle through combat.
Prerequisite: Ride 1 rank.
Benefit: Once per round when your Vehicle is hit in combat, you may attempt a Ride check (as an immediate action) to negate the hit. The hit is negated if your Ride check result is greater than the opponent's attack roll.
You gain a +2 knowhow bonus to ride checks when you are riding a Vehicle. You receive a -4 knowhow penalty to ride checks on mounts. This penalty is reduced to Zero if you have the Mounted Combat feat.
Normal: A character without this feat must pass a Knowledge (Engineering) Check, DC equal to 10 + HD of the vehicle, as a swift action to be able manoeuvre the vehicle with the ride skill.
Spells
I’ve covered that a bit already, but would love to see this section restricted somewhat, when compared to the Pathfinder RPG. We don’t need a repeat, and there are plenty of sources on line for spellcasters. What I’d like to see is the basics covered, with some healing options for the White Mage, and maybe some Jedi Mind Tricks for the Blade Adept. Nothing extensive, just enough for someone to pick up the book and run with the concepts. Later Splatbooks could flesh out magic to more detailed levels. And from a business standpoint, if there is a cry for more Jedi support, or a book for the Black Mage, then you know you are off and running.
Mounts, Vehicles and Vessels
Although I am keen to see Mecha too, I think it would be better suited to a later splat book. In fact I think for a starting point, just some basic vessels, and vehicles should be listed here. With upgrades available based on how many hardpoints each vessel has. No need to over think it, just something functional that can be tailored. IF you had 2 vehicles and 4 vessels and 10 upgrades detailed, that is at least 60 different type of craft.
WORLD BUIDLING
To me, that is the critical thing here. A Gazetteer that lets you get a feel of the worlds and space. Something along the lines of the Distant Worlds book, but with some more creatures listed in it too. I want to be able to dive in and have fun with just the book in my hands. Save space where you can, to really create a place people can visit, fall in love with, fear the effects off, and roll up some characters to call it home.
With the plethora of material already out there, this could easily be a stand along game going down a bare basics approach, which has worlds of customisation on first day of release.
I’m at a loss as to where Equipment should go into this book. Should it be its own section, or broken into the Gazetteer, with each world having unique gear, and anything that cross pollinates (on a massive scale) falls into its own section. Or should it all just go into the one place for sake of ease. I don’t know.
Note on Artwork – not a fan of what’s been shown to date. Its not at a level to what I have come to expect from Pathfinder.