First of all: Assassination. “I have one purpose: to un-alive you.”
Make them actual assassins, not the “you walk around while i DoT tf out of your friend” guy. The get-job-done guy. The specialist. The get-straight-to-the-point guy.
I mean no CC (or one thats weak, like ~7 sec blind? no stun?) The rest stays as it is: they deal high damage, as a killer should. No Vanish, cause a good proffesional doesnt leave his workplace when the job isn’t done yet. Maybe some slow to make things a little easier (caltrops style)? AoE should be potent, but on small area. I think they could branch out to 2 sub-specs: one being bleeds and poisons (DoT style), second: physical damage and debuffs (inflicting pain and making fight harder for thier target: slowing casting speed by 10%? making rage generation rng for few sec? pain that stops energy regeneration for 2 sec?) this could work like RTB and Secret Technique: attacking every target in a small area and giving few RNG debuffs to main target: slow (Achilles tendon cut), miss chance (smashed wrists), resource regen (cracked ribs), crit chance enchanced (find weakness) etc
Outlaw:
The dirty tricks guy. I mean the guy who gets you drunk for money that he pickpocketed… from you and then wants to duel you good old western style. Guy who just doesnt give a s!#t about honour, and all that matters is his well-being and his heavy coin purse.
AoE CC thats not too powerful (like ~8ft radius blind for 4 sec? gouge is good) I would like also to encourage more ranged attacks. Make him god-like in terms of AoE with mediocre ST output. I was also thinking about something rage-like: the more enemies are around him the more efficient he is (“against all odds”). They can get 2 sub-specs: one being AoE blade flurry focused with small debuffs like sand in the eyes which makes your enemies miss more and second is more of ranged (but like 15-20 ft) “keep myself safe” with almost-kamikadze “oh s$!t” button. CC should be casted often but for short time (maybe at cost of some small self debuff?) maybe lets make them forward thinking? like granade that explodes 2 sec after landing? And the most important aspect: CHANGE RTB (maybe for something that could kill your target… or you? like experimental azerite powder that can either make bullet go faster… or explode in chamber; just change it, cause rerolling so often is making me go crazy).
Subtelty:
This one would be the most engaging and difficult spec (just because i say so 
). Subtelty needs rearrangement. going in and out of shadows. I mean the know-when-and-where-to-strike guy. The guy who has good burst windows due to his almost-magical-shadow-tricks. mysterious, dark fella.
Well, in my opinion it needs to be ST. Small but long DoT’s with debuffs. Great selfbuffs that last few to -teen sec on 30-90 sec CD. Something that would make you a glass cannon, but for a short duration of time. Thier disadvantages would be thier low hp (but higher hp regen) and low auto-attack dmg. DoT’s and finishers should not waste any dmg: if you hit a mob with 2k hp left for 10k dmg then the 8k should be transited to other mob (just to compensate for almost-non-existant AoE). They could branch out to two sub spec: first with Very short, but very powerful burst windows that can occur just few times throughout fight (like focusing all shadow-power to single strike, but making it impossible to Vanish for 15 sec afterwards), above average CC that mitigated low between-burst-dmg. the second would be able to maintain Shadow Dance almost all the time, but needed full attention during rotation not to waste any CP or energy as out of shadow dance its slow to regen. The shadow dance has few charges, allowing you for some mobility and correct burst timing. one of things i would change that may be a bit drastic is to make them wear cloth: shadow dance and stealth can regenerate your life for like 7-8%/sec BUT you die very easily. thats why “magical shadow powers” grant you high protection over CC (passive, but not complete; there would be too many defensive skill that needed to be managed) and give you speed boost. no ranged attacks, just teleport to your prey and pointy-stabby it.
*by sub-specs i mean talents that can be freely mixed, but need to synergize (nothing too fancy or complicated) i may give whole propositions for talents and changes to skills… may 
what do you guys think? how to balance it out? how to buff/nerf specs/talents/skills/azerite traits/etc? how do you wish to see every spec? how would you make thier playstyle different? more enjoyable? what is your vision?