@Xypherous/Meddler/etc. Trying to think of reasonable passives that cause attackers to take damage

For a while, I've been trying to design an armor tank that damages enemies who attack it (similar to Rammus), but have been told in the past that damage-reflect passives are borderline toxic (if not full toxic). In the sense of Thornmail, I'd agree with that; Rammus gets away with it because the damage is low and only has a short window. Galio and Maokai have similar mechanics in their ultimates, but those don't immediately hurt the attacker for hurting you.

I don't like Rammus being the only Champion with built-in thorns, so I want to know if a DoT-styled thorns as a passive would be permissible.

Rough Design

Upon being struck by an enemy basic attack, your attacker takes X damage over 3 seconds. Does not stack, but the damage is increased by a percentage of your armor.

Does this still count as toxic?