If you want Cassiopeia to be played more than you should take my Advice.

except the slow is actually a powerful slow.

Nah, not really... it's just fine. People just need to learn to use the R better. Don't expect the enemy be stupid enough to just stand there and take a point-blank stun in the face.

And what are you even suggesting then? Without it being completely broken?

i always liked the idea of making cassie's ult purge like old medusa from dota1. and instead of stunning/slowing the enemy it would apply something like a cone shaped nasus slow no matter the direction.

How about when Cassiopeia ults and her enemies are facing away, she fears them for 2 seconds? That way she has some guaranteed CC on her ult to disengage but the enemy still is rewarded for not facing her.