Deadly Strike (medium set) seems like it might be good for poison focus:

(2 items) Adds 129 Weapon Damage
(3 items) Adds 833 Weapon Critical
(4 items) Adds 129 Weapon Damage
(5 items) Increase the damage your Physical, Bleed, Poison, and Disease damage over time abilities do by 20%.

And Morag Tong (medium set):

(2 items) Adds 129 Weapon Damage
(3 items) Adds 1096 Max Stamina
(4 items) Adds 129 Weapon Damage
(5 items) When you deal direct damage, you cause the enemy to take 10% more damage from all Poison Damage abilities for 5 seconds.

Or Poisonous Serpent:

(2 items) Adds 1096 Max Stamina
(3 items) Adds 129 Weapon Damage
(4 items) Adds 1096 Max Stamina
(5 items) When you deal damage with a Light or Heavy Attack against an enemy who has a Poison Damage ability on them, you have a 50% chance to deal an additional 3400 Poison Damage. This effect can occur once every 1 second.

Maybe Scavenger's Demise:

(2 items) Adds 129 Weapon Damage
(3 items) Adds 833 Weapon Critical
(4 items) Adds 833 Weapon Critical
(5 items) When you deal Critical Poison Damage to an enemy, summon the Scavenging Maw who, after 2 seconds, attacks the enemy, dealing 7210 Poison damage and inflicting Minor Vulnerability for 15 seconds, increasing their Damage Taken by 8%. This effect can occur every 10 seconds.

Possibly Swamp Raider:

(2 items) Adds 833 Weapon Critical
(3 items) Adds 129 Weapon Damage
(4 items) Adds 1096 Max Stamina
(5 items) Adds 400 Weapon Damage to your Poison and Disease Damage abilities.

I know it's probably a waste to focus poison, even with Poison Arrow and it's morphs, but it would kind of feel like a fitting theme for an Argonian, dotting up with Noxious Breath and Venomous Claw, especially with some combination (pick two) of Deadly Strike, Morag Tong, and Swamp Raider. Wouldn't be remotely a "top build" for DPS but still, it's like going on in for bleed damage. I just wish we could do more with poison on DKs.

The new Venomous Smite set would help with AoE...

(2 Items) Adds 833 Weapon Critical
(3 Items) Adds 833 Weapon Critical
(4 Items) Adds 129 Weapon Damage
(5 Items) Dealing Critical Damage inflicts Hunter's Venom on your enemy for 2241 Poison Damage to your target and nearby enemies every 1 second. This effect can occur once every 15 seconds.