Cassiopeia was more of a poison mage than anything else. Sure, 2 of her skills weren't poisons, but they helped her with her poisons/played off them. R freezes enemies, making them prime for being sitting ducks for her poison; E was something she used when her foes got too close and broke through her poisons. She was a poison mage first and foremost.
As she stands now, she isn't. She uses her "poison"to merely set up for E, which makes her too much of a one trick pony. I would love to see her be pushed back into a poison mage kit. Even if that meant making Q a single target poison, I'd be OK with that.
There are many unique things that creative designers can do to make poison mages exciting. They can make them contagious, they can make W deal more damage the longer foes stay in it, they could even have E spread the poisons onto nearby foes behind the target if it hits a poisoned foe. There's so many interesting things to do, but it would require effort and potentially reworking the kit more dramatically. If that means scrapping this passive she currently, I wouldn't be happier. It doesn't fit her at all.
