Right now, Kindred has a few feeling issues with the % current HP damage on the Passive, in my opinion.
1. Kindred is really bad after their ult; as the passive doesn't do much damage at all at 10% health, and the ratios on abilities are very low so that Kindred has no low-health burst.
2. Kindred has a hard time securing kills. Their extreme damage early in a fight causes enemies to run away more often than not, and since Kindred's chase is really bad, that means that people get away more often than not.
3. % Current HP doesn't hit the fantasy for me; Kindred is about the unavoidability of death, not about the unavoidability of staying at 100% health.
@Don Hippie suggested that the passive get changed to % missing HP, and the more that I think about this, the more that I think that it's a great idea.
My gut feeling, and one that a lot of people share on first reading this, is that it would be a buff. In a lot of ways, it is. For instance, it solves 1) and 2) above, creating more ways for Kindred to win fights and secure kills. And if Kindred is ganking a lane, it's likely that the enemy laner is already low on health (hence why you're ganking), so % missing HP is stronger.
And a lot of us still remember AD Fizz unfondly, so we're hesitant to give the green light to % missing HP on-hit.
But after considering more, I realized that this change would actually be a pure-damage (even if not an actual, in-game damage) nerf.
Consider 16-stack Kindred versus a punching bag. The punching bag has 3000 health, 100 armor, and a nearby Soraka who's going to ult just in time to save punching bag's life (+350 HP * 1.5 [below 40% HP] for 525 health). Kindred deals 300 flat damage per attack to punching bag. Here's how the very one-sided fight would go down.
% Current HP
Starts at 3000 HP
1. Deals 300 damage with passive ( 600 *.5 ) and 300 flat damage (2400 HP)
2. Deals 240 damage with passive + 300 (1860 HP)
3. Deals 186 damage with passive + 300 (1374 HP)
4. Deals 137 damage with passive + 300 (937 HP)
Soraka ults (1462 HP)
5. Deals 146 damage with passive + 300 (1016 HP)
6. Deals 101 damage with passive + 300 (615 HP)
7. Deals 61 damage with passive + 300 (254 HP)
8. Deals 25 damage with passive + 300 (-71 HP)
% Missing HP
Starts at 3000 HP
1. Deals 0 damage with passive + 300 (2700 HP)
2. Deals 30 damage with passive + 300 (2370 HP)
3. Deals 63 damage with passive + 300 (2007 HP)
4. Deals 99 damage with passive + 300 (1608 HP)
5. Deals 139 damage with passive + 300 (1169 HP)
6. Deals 183 damage with passive + 300 (686 HP)
Soraka ults (1211 HP)
7. Deals 178 damage with passive + 300 (733 HP)
8. Deals 226 damage with passive + 300 (207 HP)
9. Deals 279 damage with passive + 300 (-372 HP)
In the first simulation, the % Current HP damage dealt 1196 damage. In the second, the % Missing HP damage dealt 1197 damage. The passive deals the same amount of damage in taking a champion from 100%-0% HP. But, it still took the second attempt an extra AA to finish off the target, and a lot of the passive damage was actually dealt as overflow damage in the second example, whereas very little was dealt as overkill in the first one. You can see that heals actually increase the damage dealt by % Current HP, so fighting someone with a bunch of lifesteal or a healer around is going to be more beneficial to the % Current HP damage dealt.
But, of course, that's only theoretical damage. In game, it's often going to be that the % Missing HP would be more beneficial. For instance, after Kindred's ult, % Missing HP would let Kindred clean up everyone else extremely well. And most fights, Kindred isn't going to be engaging full-health targets, since Kindred probably shouldn't be the one engaging. And Kindred will get more money from all of those kills that were previously the enemy running away or were just assists, so Kindred will have more gold.
There are a lot of reasons to want to go with either system. Each is more powerful than the other in certain situations. Hence, a poll. Which would you prefer on Kindred, both for playing as and for playing against?
Have a great day!