You already have Insidious Roots, but you could add Chalk Outline, Amzu, Swarm's Hunger, and Syr Konrad, the Grim, to name a few
Finally, the newest Special Condition is being Burned, which acts much like being Poisoned but results in two Damage Counters being placed on the afflicted each turn instead of one
I could see an argument for Iroas, God of Victory
Ezio, Brash Novice can become an assassin with enough counters, while Haytham Kenway has protection from assassins
If you have a Commander with a brutal attack trigger, giving your threat Indestructible along with a weak creature on your opponents side, then blowing up everything else can be a huge swing in your favor
Equip Legendary Artifact - Equipment Color tag templates(None) Well of Lost Dreams Whenever you gain life, you may pay , where X is less than or equal to the amount of life you gained