Cassiopeia's passive should feel like a poison; apply an effect proportional to poison

Right now Cass's whole kit is 'how many Es can I get in,' and her passive is 'you're going to play like an adc and do more damage.' What if her passive applied an effect that ramped up with your poison damage? The more you get poisoned, the greater this effect will be, thematically making her actually feel like she's poisoning you instead of just dealing damage.

Potential effects: Draining mana/energy (anti-mage), decreased stats (you see someone poisoned? Go for the kill. Or reduced attack to make that fighter or assassin not so scary), reduced vision (little poison would be a not that noticeable vision reduction, a lot of poison would be a Graves-like smoke screen).

This could make Cass's role a 'I'm going to pick off this champ without killing him.' An assassin type we've never seen before. A new type of cc we've never seen before.

For balancing it could be her E that actually applies this effect, so the counterplay would be keep her out of range to land that E. What this would also do is create an 'o shit I have a lot of poison stacks I gotta watch out for Cass's E' for the enemy, making her E exciting even though she might not use it.

Thoughts? Should Cass's role be shifted into this pseudo-assassin mage?