Kalista E Damage Indicator
Kalista has a very steep learning curve but a strong kit. She sees a lot of play in the competitive scene but requires a lot of effort and practice to perform well on an average player. I think one of the main reasons for that is learning how much damage her E does at different points in the game, being able to actively predict when to use it for maximum efficiency. By adding an indicator in form of a color on the HP bar (both for you and the enemy), I think would improve her play and overall enjoyability potentially improving her win rate without giving her a direct buff.
By being able to see the damage outcome would also add another layer of strategy both for the player in control as well as the enemy, making it more clear when to make plays, flash away or heal/shield. I think that would make it more fun for both sides over trying to predict her damage output on the spot.
I think this would be a fairly simple and vastly beneficial change to implement.