Poisoner

Given how poorly WotC designed poisons from every perspective aside from actual use, I recommend you (or better yet, your DM) find a well developed third-party system for poisons, then adapt this class around this system. This poor innate handling of the poison system, as well as dndbeyond's strict homebrew rules, is why this class may seem fairly bloated in some parts, as there was simply too much I felt needed to be addressed for this to be a semi-functional subclass without using any third-party systems, also leaving me little room to spread out features.

Poison Harvester

When you choose this archetype at 3rd level, you gain proficiency in harvesting poisons. Add your proficiency bonus to any nature ability checks you make to harvest poisons as well as to the DC associated with harvested poisons, harvesting now takes only 5 minutes, and upon a successful harvest you acquire 3 doses of poison. In addition, once per short rest, you can give yourself advantage on any nature ability check you make to harvest poisons.

Gain resistance to poison damage.

Poison Crafter

Starting at 3rd level, you gain mastery over the creation of poisons. You now have proficiency with the poisoner's kit, craft potions at a rate of 25gp per day, and you only require 6 working hours per day which can be split up throughout the day, potentially occurring during short rests. In addition, add your proficiency bonus to the DC of your created potions.

Once you complete the crafting of a potion, you can choose to roll a d20. On a 1, the crafting fails and the poison is rendered useless. On a 19, gain a second poison of the created type. On a 20, you can choose to either gain a second poison of the created type or double the damage die and duration of the poison, as well as increasing the associated DC by 5. 

You can now apply Injury poison as a bonus action, instead of as an action, and you are immune to becoming poisoned.

Magic Poison Sacs

At 3rd level, you acquire two magical poison sacs. Choose one Inhaled and one Injury poison to magically replicate within the sacs. The chosen poisons can be used a number of times per short rest equal to your Constitution modifier (minimum of 1). One application of an Injury poison from a sac is equivalent to 3 doses (3 hits). You cannot transfer poison from within the sacs into an external container.

You can change the poison being replicated within either sac by pouring in a new poison of your choice. Doing so will also refill your uses of the corresponding sac and will add the poison to the list of available poisons you can swap to. In addition, at levels 6, 10, and 15, you can replace the poison currently in one or both sacs with a new Inhaled or Injury poison. At 20th level you can do so at the end of a long rest. The Injury poisons you can choose from depends on your level, without otherwise pouring the poison into one of your sacs. Your choices are Drow Poison or Basic Poison at 3rd level and expand to include Serpent Venom at 6th level, Wyvern Poison at 12th level, and Purple Worm Poison at 20th level.

As an action, you gain the ability to unleash an Inhaled poison in a controlled spray to a target within 10 feet in front of you. The poison then dissipates over 6 seconds, becoming harmless. The chosen poison functions as per its description. If the target fails its Constitution saving throw and the poison came from one of your sacs, the target takes an additional 1d12 poison damage. This amount increases by 1d12 at 5th level (2d12), 11th level (3d12), and 17th level (4d12).

Cloudkill

Starting at 9th level, once per long rest as an action, you can consume two uses of an Inhalation poison to create a 20-foot-radius sphere of poisonous, yellow-green fog centered on a point within 20 feet of you. The fog spreads around corners. It lasts for 5 minutes or until strong wind disperses the fog. Its area is heavily obscured.

When a creature enters the fog's area for the first time on a turn or starts its turn there, that creature must make a Constitution saving throw as normal for the poison used. The creature takes 5d8 poison damage on a failed save, or half as much damage on a successful one. Creatures are affected even if they hold their breath or don't need to breathe.

The fog moves 10 feet away from you at the start of each of your turns, rolling along the surface of the ground. The vapors, being heavier than air, sink to the lowest level of the land, even pouring down openings.

You gain an additional use of this feature at levels 10 (2), and 18 (3).

Potent Poisons

Beginning at 13th level, your poisons achieve vastly greater potency. You can use your sneak attack against a target afflicted by one of your poisons even if you don’t have advantage on the attack roll, but not if you have disadvantage on it. In addition, it is now possible for a target of one of your poisons to critically fail their Constitution saving throw. If the target rolls a 1, the effects of the poison are doubled in all aspects.

Exploit Weakness

At 17th level, you learn to exploit a poisoned creature’s weaknesses. While a creature you're fighting is poisoned, your Sneak Attack damage against that creature increases by 2d6.