Two things you could do that both work in nerfing the worst thing about Kalista.

We can all agree that the biggest problem on Kalista is easily her movement passive. A close second is her Max health in the other damage type nuke.

Solution 1 to her mobility:

Having her dash be based off of CURRENT MOVEMENT SPEED. Movement speed cap to be lowered to is currently 100? So you still have some form of mobility but the dashes still have a form of counter play.

Solution 2 to her atrocious kite:

Having her auto attack range be Morgana tier (425 or even below that) requires them to be in range and less applicable to dodge skillshots and less opportunity to be in safety, but still keeping High speed dashes.

Solution A fixes her mobility, while Solution B fixes her kite potential by giving her more risk.