Riot's mistake with Cassiopeia was their refusal to give her new abilities
So they just ended up slapping random stacking mechanics on an uninspiring 3 aoes+1 single target Mage archetype from 2010 thinking that would suddenly solve all of her problems. Vlad and Brand are honestly equal offenders to this boring archetype that riot thinks they can balance without a rework.
Huge thematic rework for Sion and various other characters giving them awesome gameplay cementing them as niche thus useful, yet when it came to the confusing noxian snake lady they just didn't put forth any effort story or gameplay wise. Sivir got a visual rework because she was confusing as a rich person that played nothing like it. Cass is a seductress and a vicious person yet why is she a poison Mage lane bully pseudo hyper carry even before the stashuification?
Poison Mage shouldn't be a lane bully. Lane bullies shouldn't have aoe zoning and clearing Hypercarries don't need stacking mechanics to be Hypercarries
Cassiopeia's design is honestly flawed and kinda boring from an archetype and thematic standpoint and maybe we need a real designer of mages like Meddler to actually work on her in the future. I'd also like Ironstylus to give her some actual indicators of style and wealth rather than half naked snake woman.

I'm probably missing some, but you get the idea.