Cassiopeia needs buffs
Yes, her late game is amazing, but unlike (most) other mages, she actually NEEDS to land a skillshot or she's not doing anything. Other mages can just press 'r' and instantly delete you (Syndra, Malzahar, Annie, etc) If she doesn't land her 'q' or her 'w' she's not getting the bonus ms (only from her q) or resets on her 'e'.
If I could compare Cassiopeia to anyone, she is very similar to Anivia. They both need to land an ability if they want to do any meaningful damage in a fight, but unlike Cassiopeia, Anivia is WAY easier to combo with. You just wall someone off, and then ult+e them or q+e them. They also both have zoning abilities, but Anivia's ice wall (whatever the ability is called) is 100 times better than Cassiopeia's miasma. TLDR Anivia is just a better champ overall. There's absolutely no reason to play her when someone like Anivia is so much easier to use.
Anyways, buff Cassiopeia please. She should be rewarded when she actually lands an ability. Make her poison last longer, give her extra ms when she lands her 'q', reduce the cooldown on her ult, increase the duration of miasma, lower her mana costs, etc. Any of these things would help her a lot. Thanks.