Hear me out, I've had a thought about Kalista.

I think it would be cool if there was a penalty of some kind to the soul bound while Kalista is on death timer to enforce the idea that YOU GAVE YOUR SOUL TO A VENGEFUL SPIRIT. That's not supposed to be a decision you make lightly, and you both get a lot of benefit out of the deal. I think some kind of "Soul-Drain" debuff that carries some stat penalties reflecting your soul being in flux as Kalista is on Death Timer would make the trade more meaningful and give you more reason to have good teamwork with Kalista.

Anyway, love it or hate it, that's my thought. Carry on.

EDIT in response to comments: Perhaps Kalista's Black Spear should be something a champion chooses and is not simply thrust upon them by Kalista. At that point, the above idea would be come relevant.