Reduce Kalista's Range to 500 from 550 and she won't be broken anymore
Now I despise Kalista just as much as the next guy, but contrary to popular belief on this forum she DOES have counterplay.
Kalista's playstyle is kind of like a dance with death, she has to stay within autoattack range to be able to use her mobility and on the other hand if someone gets too close her they can kill her. The best ways to deal with her are damage her outside of her attack range (xerath, ziggs, jayce, etc) or have access to targeted hard cc followed up by burst (usually requires coordinated team play).
So the problem right now is that Kalista's attack range is forgiving enough that she can comfortably kite champions around without ever feeling like she's in danger. By lowering her autoattack range suddenly the weaknesses inherent to her kit that have always been there but couldn't be taken advantage of are exposed and exploitable.
Of course she might need more number changes other than this, but personally I think the rest of her kit is fine the way it is. This, combined with the new black cleaver changes that have been helping a lot of ad fighters/tanks should put her in a better place.
If you disagree, state why and make your own suggestion but just try and keep the discussion productive. Saying "omg just remove kalista wtf was rito thinking" doesn't really do anything to solve the problem.
inb4 I get downvoted into oblivion for suggesting there's a way to fix Kalista other than removing her from the game