How to fix Kalista's passive
Right now Kalista has the ability to effortlessly kite 95% of the melees in the game and since melee accounts for atleast 40% of the champions in the game this should not be. The problem is her passive, even if she has 99% slow (yeah talon does this) she can just dash away even though she is slowed. With enough attack speed (a natural choice for a AD carry like Kalista) it becomes nearly impossible for melee to keep up with her. "But she has a high skill level!", that hardly matters because as soon as someone figures her out she quickly becomes unstoppable.
I'm already upset with Kalista in general, what with her FORCED oath and her ultimate that is either awesome or terrible. as well as the fact she can do %-based damage at level one that scales with max hp making it very difficult to counter (yeah I don't like vayne for this reason either).
But to address the most troubling aspect of Kalista, her passive Martial poise, all that is required is disable her passive whenever she is effected by some form of crowd control (kind of like how Miss Fortune's passive works); this would enable melee to have a chance at catching up with her but only if they gap close to apply a form of CC. A crafty Kalista can still evade CC if they dodge well enough, this allows for a fluid situation where the victory goes either way instead of having Kalista run away with a 99% slow like a demented jackrabbit.