Kalista seems like she needs usability buffs first-

-rather than straight power buffs since it's hard to tell how much of her performance issues are due to her difficulty rather than her actual raw power.

So how about this, make her heap of spears lodged in a person change color or glow or something if there are enough of them stacked up to execute a foe when she casts Rend. That would make it a bit easier to tell when to drop the nuke so she doesn't lose about a fourth of her damage output because it would have taken one more stack to finish someone off.

Because people would be able to use Rend in a more optimal fashion, it would make it much easier to assess Kalista's power when she's being played to a higher level of potential.