I spend way more time tinkering with systems than I actually get to play. With two young boys at home, a full work schedule (for both myself and my wife), I tend to get most of my gaming fix either through video games or tinkering with table-top games.
Like many gamers, I have my favorite system for each genre I enjoy. Swords & Wizardry (though I have hacked it up quite a bit and I'm constantly adding more material to it) for fantasy, Marvel Superheroes (TSR) for supers, Boot Hill 2nd Edition or Savage Worlds for Westerns, Savage Worlds for modern games, and Stars Without Number for Sci-Fi. However even though I enjoy most of the mechanics of SWN, I'm not a fan of the setting, and there are things mechanically I would do differently, with those reasons playing a big part in the tinkering around with hacking SWN into a system a bit more palpable with my personal tastes for a sci-fi RPG.
As I was working on skills for my SWN hack, I found that I just wasn't crazy about how skills are set up and even got to the point that the three classes left me wanting. I've long been a big fan of Alternity. I played it for several years before getting swept up in the D20 wave. I like the professions in Alternity and got to playing around with the idea of porting the basic profession structure into an old school framework. What I ended up with was 5 classes instead of three, and a bigger emphasis on skills. Here's what I've got so far:
Warrior: These are the soldiers, bounty hunters, mercenaries, security forces, and "fighters" of the galaxy. Their skills are largely based on combat (weapons, armor, martial arts, etc.) and they are the toughest of the tough. They get a +3 bonus to Initiative, Veteran's Luck special ability (as found in the warrior class in the SWN core rules), +1 bonus to skill rolls to one selected skill from the following list (list not set in stone yet, so it will come later).
Freelancer: Freelancers are the jack-of-all-trades kind of characters. They rely on guile, skill, and cunning. They are capable fighters, but not as proficient as warriors. They have access to a wide array of skills, but tend to have a wide array of skills, and therefore are not masters like Experts. They get a +2 bonus to Initiative, Like a Charm special ability (as found in the Expert class in the SWN core rules), and a +1 bonus to all saving throws from any one saving throw category.
Diplomat: Diplomats are the deal-brokers, ambassadors, and "face-men" of the galaxy. They use their charm and personality to sway others to do what they desire...for both good and ill. +1 bonus to Initiative, Speaker's Voice special ability (as found in the Speaker class in Other Dust), Secondary Profession: diplomats pick one of the other classes as their secondary profession and can purchase skills from that other class' skill list without having to pay the cross-class price).
Expert: Experts are the technicians, scientists, engineers, and other mechanical/techinical/medical specialists in the galaxy. Their skill list is heavy on knowledge and technical skills, but light on just about everything else. +1 bonus to Initiative, Accelerated Learning (bonus skill points per level that increase as the character gains levels), Flawless Skill special ability (as found in the Scrounger class in Other Dust).
Psion: Psions are those that have unlocked the potential that many species have to tap into mental energies to affect the world around them. They are both feared and awed throughout the galaxy. +1 bonus to Initiative, Can learn psionic powers, Choose one psionic category and gain a +1 bonus to activation rolls to powers in that category (yes, this would mean a change to the Psioncs rules posted here)
I'm considering going with skills in the style of Alternity as well, with expensive broad skills, but cheaper and more precise specialty skills, but I may also just use the skill list I already have generated here, but with some possible tweaks. If I did go with the Broad/Specialty format, I would likely give each species some suggestion broad skills that fit their backgrounds. I do know that all characters would get a certain amount of skill points (with a bonus for higher Intelligence scores) and would get more skill points each level. This would allow characters to branch out a bit more.
Now, all of this is still in the very early stages of planning, more akin to brainstorming than anything else, and may very well change when I get down to actually statting everything up and running the numbers. It's also very possible that I may just scrap the entire idea altogether.