A fix for the assassin, due to the incredibly lackluster later abilities, and the incredibly niche use of it's main ability.

Bonus Proficiencies

When you choose this archetype at 3rd level, you gain proficiency with the disguise kit and the poisoner's kit. If you already have proficiency in these tools you may choose a new tool proficiency, or gain expertise in these tools.

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 is unaware of your presence or hasn’t taken a turn in the combat yet. In addition, any hit you score against that creature is a critical hit.

Quick Draw

You may draw and make an attack with a weapon in the same action so long as that weapon has the light property.
If you make a weapon attack during your turn you may draw and attack with another weapon as a bonus action on the same turn, so long as the weapon has the light property.
The weapon(s) may be thrown if it has the thrown property.
As well, you may add half your proficiency bonus to initiative checks (minimum of 1).

Infiltration Expertise

Starting at 9th level you have advantage to Deception checks when using a Disguise Kit and on any check needed for checks for making and using a false identity.

Thereafter, if you adopt the new identity as a disguise, other creatures believe you to be that person until given an obvious reason not to.

Deadly Flourish

When you hit with a melee attack the target of your attack may not use their Reaction to make an opportunity attack against you.
On a kill, you may automatically make an Intimidation check against a number of creatures within 15ft of you equal to your Charisma Bonus.
Each creature that is successfully Intimidated gains the Frightened condition until the end of your next turn.

Impostor

At 13th level, you gain the ability to unerringly mimic another person’s speech, writing, and behavior. You must spend at least three hours studying these three components of the person’s behavior, listening to speech, examining handwriting, and observing mannerisms.

Your ruse is indiscernible to the casual observer. If a wary creature suspects something is amiss, you have advantage on any Charisma (Deception) check you make to avoid detection.

Expert Poisoner

You may apply poison to your weapon as a Bonus Action.

You learn how to make a basic poison quickly and for cheap. You can make 3 vials of basic poison during a Long Rest for a cost of 5 gold.

The Basic Poison has a DC of (savedc:int)  versus the target's Constitution save. On a failed save it deals 1d4+INT poison damage and applies the Poisoned condition, on a successful save it deals 1d4 poison damage with no additional effects.

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.