[Yashatari]

First, Void Assassin. You have to main it to even be able to use it properly, As voidveil is locked behind that [main class only] barrier.
http://prntscr.com/j61685 For reference.
This cuts down on its uses and versatility. I had an interesting build with it, But it was swiftly eliminated when i saw this.
This is the only class like this, where one of its main features is locked like this.
Kensei, DH, Mages, Archers, Monks, Basically everything else doesnt and only a few skills are locked like that, But they can still work as sub classes, This is not true with VA. Enough about this...

My second complaint, DH has cobra and the only ways around are having high skill or multi hits, It makes certain bosses a joke.
It has very few rival builds as it can dodge what normally cant be. It's stupidly OP even when compared to Iesep body, That has to be reapplied every round or so, But not cobra as it reapplies automatically.
I personally think this can be remedied if every class had at least 1 Multihit skill, But those are rare skills.
All mage skills are single hit, So Cobra'd. Duelist/Kensei, Cobra'd. Unless you're a soldier using a spear or axe, or a gunner or even a rogue with dagger dance, There are very few ways around this other than move/attack, or absurdly high hit/autos.
I've seen to many meta builds with DH in it.

[Rendar]

Cobra is.. really a non-issue now-a-days in regards to it.

You have to make an evasion check vs auto-hits (which requires you to have evade)..

There are tons of movement skills that get around watchful eye / move you..

And you can also just. Y"know. Move to their back. Blind / immobilize stop it as well. So you can just slap them with blind through any of the many ways and be done with it.

Void Assassin is pretty useless now, because there was DR% stuff that was being abused, so Dev just gated it behind mainclass, pretty much killing the main class.. Which solved the issue. I guess.

[Snake]

It didn't solve anything, just kept us from seeing Void Assassin ever again in the game.

[Yashatari]

At Rendar.
Thing is, I'm not concerned about the DR. I had an idea planned for VA regarding the backstabbing traits of rogue and VA. I wanted to use it for Damage purposes, Hell even the crit from viel off and blotch for the hit/evade. But as you can guess 80% of that class is locked behind mainclass, So imo its effectivly the worst class now. And the DR only really works against magic anyway, and 15% still isnt as bad as the classes like boxer that had 25%. Honestly, Its mainclass restriction should be removed by this point.

[Lolzytripd]

Could be solved by making void veils Dr 50% weaker when subclassed.

[Snake]

A few things being removed from main class only will certainly bring back Void Assassin, in my opinion. But many might just disagree. My idea about what should or not be locked away in VA are:

Voidveil shouldn't be main class only. Voidveil's damage reduction effects should be halved if sub-classed.
Detogate should be main class only.
Black Bolt should be main class only.
Dagger Dance shouldn't be main class only. (And I say Dagger Dance because it only really interests VA, in the end of all.)

Void Assassin, as a class, doesn't offer much or enough to have the MC treatment. If only it had a hundred more skills like Spellthief or Demon Hunter does, it would warrant it, because you would have more options to pick, even if that is sub-class. But sadly, that's not the case, for a class with only like ten skills added to it, and one being a passive that works more like an innate.

[Autumn]

As Kunai brought up:
-Voidveil shouldn't be mainclass.
-Dagger Dance not being main class would probably immensely help the class' damage.

And my own:

-Deadly Arms can maybe be buffed in some sense
-Cutthroat should give hit for its FP cost now.

And that's really about it, changing these 4 skills will bring Void Assassin back to relevance I feel.