So what if you gave Cassiopeia a rip-off of Viktor's old Passive and allowed her to choose?

Upgrade her poison abilities so her poison can do jack. Upgrade her E if you feel like playing a pseduo-marksmen but some guy called "AD bot or feed" in champ select. As added bonus you removed her thematically backwards passive that artificially gates her for no reason too!

Now you may be wondering, "Wait! why the hell does Cassiopeia have such a random and unfitting Passive?! How does that fit her thematically at all?!" But by this point of the rework, serious champion disjunction is hardly an issue for you now is it? Might as well go the mile so at the very least Cass players can have what they want. All her abilities can be shitty half-abilities like her Q W currently are (cept now it extends to her E too!)

If that doesn't work out, do what you did to make people like this rework. Just keep throwing numbers into it till she becomes viable or OP.

If you're a Spike, here

http://cartoon-coloring.com/images/59816-cartoon-snake-clip-art.jpg

If you're not a Spike,

http://img1.wikia.nocookie.net/__cb20140830162228/pokemon/images/thumb/6/66/Jessie_Ekans_Acid.png/500px-Jessie_Ekans_Acid.png

Both sides win.

(on a side note, I can not BELIEVE that image actually exists!)