Core mechanics:

Skills apply exposure stacks (max 8 stacks)
At 8 stacks the target becomes saturated for 4 seconds (slowed or whatever effect is cool here) - Can be dispelled
Some skills consumes exposure for bursts, cc and/or spread to nearby enemies.

Tree 1: Corrosion (Offensive)

1: Caustic Needle

Skillshot, Applies 2 exposure and a short DoT
If skillshot hits a saturated target it also "splashes" 1 exposure to nearby enemies.

2: Septic Patch

Throw a vial that crates a corrosive ground zone (stays for x amount of time
Enemies inside gain exposure over time and take reduced healing while standing in it.

3: Volatile reaction

Detonate a targets expsure, consumes up to 8 stacks for burst damage
Bonus effect, if you consume 8 stacks, leaves a small acid pool on ground (that dmgs anyone in it, except friendly)

4: Organ melt

A short channel beam
Deals steady dmg and ramps up faster per exposure stacks on the target

5: ULTIMATE(!!!!) Black death

Large drifting cloud that you steer slowly
Applies exposure very rapidly, lightly blinds (reduced vison) which punishes clumped teams.

Tree 2 - Antiserum (Supportive)

1: immunization spray

Cone spray that heals allies in a small area
Grants immunization (short shield)

2: Neutrlize

Single-target cleanse on an ally.
Converts removed debuffs into reagent charges (next heal or shield is stronger)

3: Toxin siphon

Mark an enemy a % of the poison damage you deal to them, heals the lowest health nearby ally.
If the marked enemy becomes saturated, the healing spikes briefly.

4: Serum vard

Place a small ward zone
Allies take less dmg (+ other stuff?)
Enemies inside gain exposure faster.

5: ULTIMATE!!!

Team-wide pulse: cleanses and burst heal
For x seconds after, allies gain a "antidote shield" (first debuff they recieve is reduced like it´s partially resisted)

Tree 3 - Mutation (Utility / mobility / dirty tricks )

1: Ooze Form

Transform into a toxic uddle for x seconds, move faster, ignore unit collision, can slip through tight gaps (level design needed)
Leaves a trail that lightly slows enemies.
You can´t attack while a ooze form, re-emerging causes a smal splash applying 1 expsoure.

2: Molt Decoy

Shed your skin, creates a decoy and gain stealth
The decoy pops after a delay or when hit, applying exposure in a small radius.

3: Spore mines

Toss two mines that arm rather quickly
Triggered mines applies a short slow and adds exposure.
Mines can also activate on invisible players (future classes)

4: Venom Lash

Mid-range tethering skillshot (if you wanna give skillshot a try)
If it hits an enemy; drives you towards them (as a gap closer) or if you aim at a terrain it will pull you to the anchor point.
If the target is saturated, they are briefly rooted in place.

5: Ultimate (EWMAJGAD!!!) Ebb and flow

Place two toxin pools for x seconds (example 8-10 seconds) - Very much like your rally skill
YOu and allies can dive into one pool and surface from the other, not instant but short travel time here.
Enemies who try to cross through a pool are slowed heavily and gain fast exposure for x seconds.
This is a teamfight macro move for engages but also saves. Like teleport but ... venomwright style!

So TLDR;
Not a burst-first, it wins by stack pressure, space control and PUNISHING clumps!
Support tree isnt a holy fuck paladin healer but a chem-support that cleans and turns poison into value and last, weird puddle stuff :D