An entire season has passed and the Cass rework still hasn't met its goals.
The original purpose of the rework was to create an overall more accessible and attractive pick for players, however it didn't fullfill any of those goals. Instead it removed any build- or gameplay diversity the champion once had and reduced her identity to the one of a squishy, immobile, short ranged ap marks(wo)man with crippling manacosts.
Meanwhile her identity as a poison mage was deleted ENTIRELY thus killing off the Cassiopeia we once knew and loved. Yes, the one champion who was capable of kiting and melting entire teams with well placed miasmas and noxious blasts alone. Nowadays her poison can't even kill minions anymore, she has become a shadow of her former self.
Since then she has become a lot worse: prerework Cassio dealt 50-60k (!) per game with her aoe-poison damage alone while staying relevant throughout the game thanks to her great ratios and flat numbers. Only a small part of her damage came from spamming Twin Fangs since it was an extremely risky ability to use in the first place, only meant to execute those who were greatly weakened.
Now she has to wait 35-40 minutes before she even gets to deal a fraction of the damage she used to do. And even then it's extremely unsafe for her since it requires her to go very deep into the dangerzone. And it gets a lot worse during teamfights, trust me.
Nowadays she's supposed to play like an adc, and yet she lacks the support to peel for her once things get hot. She simply can't survive on her own, it's just not possible. The fact that you expect her to get close enough just to get jumped upon reduces her window of opportunities by an extremely large margin, this isn't the kit of a kiter anymore. It's a deathwish.
Literally ALL of her numbers were nerfed - Cassiopeia's ratios and flat magic damage were drastically reduced while essentially DOUBLING the manacosts after she lost her old passive which originally was the greatest passive you could have asked for as a spammy mage. You still expect her to keep up with the Twin Fang spam, it's just too bad she doesn't have the mana to do it anymore.
Even with a Tear she still struggles at keeping up with lane activity or at taking down raidmonsters. Reducing the total number of Twin Fangs she can cast before going oom is by far the biggest nerf to her overall damage potential. How could you have overlooked such a massive flaw?
Her entire play pattern is the epitome of high risk and low reward since anyone can shut her down instantly if you have access to cc and/or burst. Especially during lategame, where she's supposed to shine, she blows up as fast as her targets. The little bit of sustain you've given her really makes no difference at that point. Heck, just look at the attached imagefile if you want to know how insignficant it is.
Why do you force us to spend the majority of the game as an obsolete champion who struggles to even keep up with every other midlaner just to become irrelevant again once lategame rolls in? There isn't a single matchup anymore that she could even reliably win, it's just nonstop farming and sitting at your tower. Considering her massive handicaps, she never gets into a position that would enable her to 1v5 - it's downright unthinkable.
The most blatant issue however has to be this new role you had in mind for her. She's supposed to be THE ap-hypercarry in the game and yet her performance falls drastically short of achieving that. The numbers virtually speak for themselves. If she can't properly do her job, she's has to be declared unviable - it's that simple. It's beyond me why you haven't attempted any further changes on her so far.
Several other champions leave her in the dust when it comes to dealing tons of damage. Kog'maw, Karthus and even Teemo (!) deal 10k damage more than her on an average basis. Seriously, there are over 40 other champions in this game who deal more than Cass. Her overall damage doesn't even come close to what assassins put out in the course of a single game, it's simply disappointing.
At least back then I was able to feel good about the damage I dealt to my enemies whenever I looked at the endgame-chart, now it's just one disappointment after another once I see the results. I've yet to play a single game with this reworked Cassiopeia where I actually deal the most damage in the game. It simply doesn't happen anymore and it makes me feel extremely weak.
Before the rework she used to be at a stable 3% pick- and 50% winrate. Now she's down to a mere 2% pickrate while sporting a 46% winrate on average basis. She has become a worse champion in every single regard, and yet you refuse to keep working on her. Even the guy who originally was in charge of her rework left this sinking ship over half a year ago, since then no one even felt obliged to take care of her. That needs to change, Riot.
I think it's about time you finally give us back the identity she once had, as a poison mage befitting the seductress she was born to be.
These items ensure she's decently durable.
Issue is, she has the same problems Veigar has; I'm pretty sure her power level isn't the problem.
It's a problem of gameplay fantasy
No one played Cassiopeia to be a Twin-Fang machine gun
She's supposed to be the premier DoT mage
(yes, WOTA is decent on Cass and definitely viable) 
is love