To me, the original Assassin subclass always felt a bit like two ornately crafted bookends on either side of a row of grilled cheese sandwiches. Both lovely things on their own, but not really suited to work together. The subterfuge mechanics may have a place in some campaigns, but can be replaced by a small multiclass or additional ASI option. Furthermore, the base Rogue already has great utility and I don't really think about needing more utility when I hear the word "Assassin."

So, here's an edit to alter the combat traits of the Assassin Rogue.

• Replaces disguise kit proficiency with a stripped down "Poisoner" feat. Doesn't ignore resistances, but it's damage and DC does scale with rogue level. It will be less useful as more poison resistant/immune baddies show up, but it can be supplemented with the actual "Poisoner" feat to add more punch.
• Replaces "Infiltration Expertise" with the ability to add proficiency bonus to initiative. Paired with Reliable Talent that means the Assassin usually gets to act first and gets to more regularly use "Assassinate." It's very powerful, but it's still only 1 round of unbridled joy, and it prevents a single bad roll from negating the biggest trait of this class.
• Replaces "Imposter" with "Up In Smoke." A number of times per day equal to the rogue's proficiency bonus they can deploy a smoke bomb (centered on them) whenever they take the disengage action. It makes a heavily obscured 10 ft. radius sphere centered on them until the start of their next turn. This can screen an escape, or grant advantage on attack rolls against an enemy that relies on sight within the sphere as the Rogue is not considered blinded by their own smoke.

Potent Poisons

When you choose this archetype at 3rd level, you gain the following benefits:

• You gain proficiency with the poisoner’s kit.
• You can now use your cunning action to apply poison as a bonus action.
• All good artists sign their work, and you've developed your own signature. You learn a recipe for a potent, signature poison. At the end of a long rest, you can create a number of doses of this poison equal to your proficiency bonus. Once applied to a weapon or piece of ammunition, the poison retains its potency for 1 minute or until you hit with the weapon or ammunition. When a creature takes damage from the coated weapon or ammunition, that creature must succeed Constitution saving throw against your DC ( 8 + your proficiency bonus + your Dexterity modifier) or take 2d4 + your rogue level in poison damage and become poisoned until the end of your next turn. 

Assassinate

Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.

Ambusher's Instincts

You have a preternatural instinct to make the first (and preferably only) strike. You now apply your proficiency bonus to any Initiative check that you make. These checks can benefit from your Reliable Talent feature.

Up In Smoke

"POCKET SAND!" - Anonymous Assassin before fleeing

You've turned your alchemical talents making your escapes (possibly) more effective and definitely more dramatic. When you take the Disengage Action either as an action or as a bonus action you can choose to throw an instantaneously activated smoke bomb at your feet. This generates a cloud of dense smoke in a 10 foot radius sphere centered on you until the start of your next turn. To creatures other than you this area is heavily obscured, but it does not impose those penalties on you- you have a crystal clear mental image of the area before the smoke.

You can use this feature a number of times per day equal to your proficiency bonus and regain all expended uses after you complete a long rest.

Death Strike

Starting at 17th level, you become a master of instant death. When you attack and hit a creature that is surprised, it must make a Constitution saving throw (DC 8 + your Dexterity modifier + your proficiency bonus). On a failed save, double the damage of your attack against the creature.