Tried the new Cassio in a custom against my friend a few times, and so far it's not as bad as I expected, but there's quite a few issues I experienced.
First of is the poison duration on her Q. 3 seconds, while the cd of it is 3.5, meaning you tend to hit one of your twin fangs without the bonus damage. Why not make it 3.5 seconds so you can at least keep enemies poisoned as long as you hit them?
So now the twin fang, I kind of understand why it scales per level but common... At the first few levels it deals less damage then your basic attack if the target isn't poisoned. And that is the main problem with this skill. If you cast it without the target being poisoned you might as well have done nothing, as the damage is far too low. Even last-hitting is a pain in the ass. Maybe allow it to deal 50% bonus damage against minions, and make the early game damage higher while reducing the poison damage early to compensate. I do like the damage you get late game though, but I still prefer the old one as it was more forgiving for not hitting a poisoned target. 4 seconds of not doing anything VS 4 seconds of doing nothing but still wasting over 300 mana.
Then there's that miasma. Grounded stuff is great, too bad it has a minimum range. And that makes this ability a piece of junk. With all mobility creep going on, a Zed buff and all champions that can jump on top of you even now with that rocketbelt. Unless you have your ult you are completely defenseless. Why not allow me to cast it underneath me so I at least have a small chance to get away?
It used to be a reliable way to poison your enemies, even if they were mobile. Now there's just your Q left which doesn't make up for it. And your twin fang won't save your ass as the damage it deals is almost negative.
Ult pretty much unchanged, but you could have added a poison on that too to make up for that miasma.
The passive is fine to me, it saves a slot and since the Alacrity enchantmant can't be bought you don't really miss anything as you can cover the stats with other and better items. Just don't bring back the boot enchants.
(The passive is also boring as hell IMO, kind of like Renekton's)