Kalista Oathsworn/Rend Swap
This is at least the 3rd time I have scrapped and restarted this topic because I feel like I'm getting a little too wordy and complicated. So I am going to try and keep it simple and hope that everyone understands that any major changes to a champion would no doubt require some more in depth, detailed re-balancing as well.
So simply this
Get rid of the unnecessary 
Make the Oathsworn passive an ability like Taric's Bastion ability.
Move Rend to her passive.
After her 5th spear lands, including spears from her Q, all spears are automatically rend. Damage is amplified by the number of spears in a single target.
Again, keep in mind that there I absolutely expect and understand that there would be balance considerations needed to be taken beyond these changes.
If anyone even likes this idea, I can try to get more depth into those things and make a more detailed post.