If You Are Going to Stay With the Current Cassiopeia Kit...
She needs a couple adjustments. These aren't major, but are enough to be noticed.
1. Her Passive Needs to Feel Rewarding Immediately This is a big thing. Her passive should make you feel more powerful EVERYTIME you gain a couple of stacks of it, not every 100 stacks or so. The reason it feels like a chore to a few is because you are working and for a good while it feels like you are not gaining anything from your hard work. Make you feel that passive power immediately, like every 15 or so stacks you get some extra AP.
2. Buff her Q Ratio Make it around 55% or 60%. 45% is laughable and make it hardly benefit from the one thing Cass will have plenty off... Ability Power.
3. Buff her E BASE Damage 10/15/20/25/30 base damage is incredibly bad for how the ability works (over 7 seconds AND is avoidable). Increase the base damage and make the skill less of a joke. (I am thinking 15/25/35/45/55 or 25/30/35/40/45 personally. Even a little buff like this will buff it enough due to the E poison debuff and since the damage gained can be multiplied by seven since the area lasts for 7 seconds)