Does Cass need a new Ult?
I feel as though Petrifying Gaze isn't really all that great a fit on Cass, the supposed "poison mage". She can petrify people with her gaze. Why? Because it's a reference to Medusa? Not particularly original, or even interesting.
Should Riot give her something else? Maybe like:
Fatal Attraction: Cassiopeia channels for two seconds then leaps at target champion. If she reaches her target she suppresses them for three seconds and deals 300/400/600 (+1.5 AP) over the duration. (Can be stopped with CC.)