@Riot, PLEASE rethink your approach on Cassiopeia
First of all, it's good to see you boys and girls at Riot haven't forgotten Cassiopeia entirely. However I fear the way you're trying to go with this new Cass is a VERY questionable one.
I'll try to avoid bombarding you with numbers and mathematical abstractions, but let's take a look at what we're dealing with.
PASSIVE: Aspect of the Serpent Cassiopeia gains a stack of Aspect of the Serpent for each second that an enemy champion is poisoned and for each poisoned unit that she kills. Her ability power is increased by 1% for every 25 stacks, up to a max of 375 stacks.
At 50 stacks, Twin Fang now heals Cassiopeia for 6/8/10/12/16 (+0.1 ability power) At 150 stacks, Cassioepeia gains 30% CDR At 375 stacks, the AP bonus from Aspect of the Serpent is doubled.
The sheer content you've put into that passive might seem quite awesome at first but let's remember what purpose this passive really serves. It's about restricting Cassiopeia's sheer power, one which the current one already has full access to from the very first level due to her ratios. This version of Cass literally has no early or midgame, and once she falls behind she becomes irrelevant.
It takes at least 6 (!) minutes of NONSTOP poisoning to stack that passive, the only way to acquire stacks is by killing minions or by hitting champions with your delayed skillshots - so good luck with that. I also don't see how free cdr could be good on her, it's a wasted stat which already negates the purchase of any other items that offer cdr.
The spellvamp on her E is quite pathetic and will do very little to raise her survivability. She's still terribly squishy and lacks any reliable escape-tools. You seriously underestimate the power of gapclosers - they offer a tool to instantly relocate your champion which is as valuable as the summoner spell Flash itself.
Q - Noxious Blast Provides Cassiopeia more movement speed later in the game but, in turn, deals less damage. Damage: 70/105/140/175/210 (+0.35 ability power) Movement speed: 10/15/20/25/30% Cooldown: 4 seconds Mana cost: 40/45/50/55/60 Delay increased by 0.05 seconds
This has to be among the biggest nerfs she recieved. Cassiopeia is a DPS mage, she relies on all of her abilities to deal a constant flow of damage, especially her Q when it comes to securing your lane presence. The great 0.8 ap-ratio which this ability once possessed is gone for good and the flat damage has been reduced to a laughable poke.
The mere 5% bonus on the speed buff won't make a difference since ANYONE with a gapcloser can instantly reach you before you even get to slither away. The increased cooldown makes things even worse - you can't even spam the ability anymore. And why on earth did you increase its delay without even expanding the radius of the hitbox?
Overall this is a massive punch into the face of anyone who would even consider leveling her Q first since it defines an entirely different playstyle for her. We already know how limited the range of her Twin Fang is but this literally forces her to stand in the face of her enemies to even deal damage due to how irrelevant this ability has become.
W - Miasma Now a notably more powerful slow tool but has a higher cooldown and deals less damage. Damage per second: 10/15/20/25/30 (+0.1 ability power) Cooldown: 15 seconds Mana cost: 40/50/60/70/80 Slow: 15/22.5/30/37.5/45%
The slow is laughably weak and only becomes apparent in the lategame since NO ONE is ever going to level W over Q or E first. In return you've REMOVED any damage she had on that ability. How is she supposed to melt down enemies during teamfights if her only reliable source of damage has been limited to her E? You've reduced her overall DPS massively with this change which was pretty much the only reason to play her.
With the loss of her status as DPS-queen you'd be lucky enough for anyone to pick her over Karthus who has FAR more reliable damage output and still has amazing AOE. Not only did you gut her laning phase by imposing those awful damage nerfs on her early and migame, no, you even nerfed her teamfighting ability. What purpose does she even serve now?
E - Twin Fang Is now MUCH more reliable when checking for poisoned targets and has a higher AP ratio later in the game. Also, now grants mana upon killing a unit. Damage: 55/80/105/130/155 (+0.4/0.45/0.5/0.55/0.6 ability power) Mana cost: 30/45/60/75/90 Utility: Now refunds its mana cost plus an additional 3% of Cassiopeia's total mana when it kills a unit. Usability: Now has additional checks in place to see if target was poisoned.
What you're trying to sell to us as a buff should have been in the game from the very beginning, it's beyond me what took you so long to fix the poison detection on her E. The mana-refund is the ONLY redeemable factor in this.
However she'll STILL run out of mana insanely quick if she's forced to commit. Can you calculate how much mana it costs just to land the right number of Twin Fangs to finish your opponent off with this horribly weakened Cassiopeia, especially in the early and midgame? I doubt you'll get the chance to regenerate any mana while fighting for your life.
Your attention is entirely on the enemy champion and not on their minions. Surprise, you're all out of mana while your opponent still gets to stay in lane. She absolutely CANNOT fight once her mana falls under a certain amount since the ability is never going to get cheaper unlike for the old Cass thanks to Deadly Cadence.
Not only did you lower the overall damage on this ability (it used to cap at 190 damage), you even felt like slapping a scaling ap-ratio onto it just to limit its effectiveness even further (she originally had 0.55% on all levels). This is a HUGE nerf to her early damage which pretty much makes it impossible to get that early first blood you desperately need to secure your lane dominance.
R - Petrifying Gaze Lower cooldown, but also slightly lower damage. Damage: 150/250/350 (+0.5 ability power) Cooldown: 120/110/100
Now this has to be the most unreliable ultimate this game has ever seen. Instead of improving the terrible delay, the short range or even the awful hitbox which makes you automatically miss ANYONE who even is capable of jumping onto you, you settle on a flat cooldown reduction of 10 seconds and call it a day. Oh, and you also removed even more damage from her kit. If this thing doesn't off your enemy, you are DEAD. Which will happen a LOT more often from now on.
Overall I'm quite disappointed at the way you've handled this rework. You took away so much of her power while limiting her build options massively (you NEED to build her as a glass cannon or you will deal NO damage). Her abilities leave no room for other playstyles anymore, infact she's even riskier to play now (your only source of damage being your E).
The risk/reward ratio is entirely skewed since you will instantly blow up in any teamfight and should you ever fall behind during your laning phase, you're pretty much shut down for the remainder of the game. No chance of coming back, period.
I don't see how those changes are going to raise her viability in any way. She'll be still left unplayed since there are far superior hypercarries to pick from who are also a lot safer to play as. I'm sorry to say this but there is absolutely no reason to play Cass anymore.

and no cdr, but your missing out on 
would be cool. maybe even a tankier version
