My Kalista rework
I thought it would be fun to do this since she won't be seeing a full on rework anytime soon.
Foremost: The item she begins with to soulbound her targets should be a trinket similar to Rengar's. I'm uncertain how it would remain functioning as a trinket.... but I believe it should have the ability to change her soulbound throughout stages of the game with the changes to the ult I'd propose.
Passive- Changed to every 2-3 basic attacks. Start her out with every 3 attacks, then at 9 make it 2.... 18 revert it to after every AA since the game has gone on this long. She is still able to absolutely -shit- on every adc player's ability to stutter step, but now she has to put more thought into. Like a good vayne keeps track of silver bullet procs.
Q- It's completely fine as it is. Perhaps an additional effect like "Kalista's next basic attack will proc her passive regardless" Maybe even make it a short lasting buff (Like lucians) that is consumed only when she wouldn't normally get a free dash after her auto attack.
W- It's okay as it is.
E- The slowing effect should scale slower than it does with each skillpoint put into the ability. There also needs to be a cap on how many stacks she can get on dragon and baron. An adc with this kind of mobility, scouting and potential cc and infinitely stacking nuke on champs she can potentially train down.... doesn't need objective control that has the potential to be equivalent to smite... much less exceed smites threshold.
R- Completely trashcan the original ult as it will always remain a tool to grief fellow teammates. It is the only interaction with her soulbound outside of proccing W, so replacing it needs to fit that theme.
I would propose a sort of skillshot that originates from the hitbox of her soulbound, but aimed by the summoner who controls Kalista. She will now be taking advantage of her soulbounds positioning to land this ability and nothing more. If the soulbound is ranged then this ability gains range with less of a knockup time. If the soulbound is melee then it would have shorter range with a greater knockup effect. Risking her soulbound against there will is not something that can be improved upon. That's complete and utter trash.
That's how I would rework her into what I think she should have been.
TL;DR She doesn't need this much mobility at level 1 tied to her passive. She doesn't need objective control with the other things her kit already offers her. She shouldn't be able to CC or move the soulbound from position. She needs to be taking advantage of the positioning of her soulbound and nothing more to fufill the "Coordination with her soulbound" theme.