Ideas to tune down Kalista's mobility
Having an unstoppable auto attack doesn't mean squat as soon as she gets attack speed. It becomes a joke to fight her as a melee champion. In a perfectly balanced game, every champion, no matter if they get countered, should be able to have at least some room to outplay his enemy and at least not get destroyed. Kalista completely poops on melee champions as well as skill shots and it's just bad gameplay to feel useless against her. Every kiting mechanic should be limited somehow by cooldowns or other limitations.
Her passive could go something like this
She can dash only when she Q, procs W's passive and uses E which is nice because it has a cooldown reset. That is 3 dashes in 8 seconds (max rank abilities).
Alternatively you could even do something like this: Each time Kalista uses Q, procs W's passive or uses E she stores a charge of Martial Poise (up to a max) which allows her to dash on her next auto attack.
Reduce dash range when slowed
An idea from another post: Make her dashes smaller when dashing towards a direction that isn't her soul bound ally
Let me know what you think and if you have any ideas yourselves!