She needs changes, like a mini-rework, not just a straight out buff. COnsider people who have to play against her, not just the ones playing her.
Anyway, even if she did have a slow cleanse, her E would be the worst skill to have it. Her E can affect multiple targets, reset (which, with a slow cleanse, would be annoying AF in lane), not to mention it already applies a slow to targets.
The only way I could possibly see a slow cleanse on Kalista is if she were to take
, or it were applied to her Q or something, but we shouldn't overload her signature ability even further.
When a frustrating champion has a weakness to counter what makes it frustrating, the last thing you want to do is give them a bandaid to completely negate it.