Sion's passive punishes him for gaining levels
Sion is intended to be a hyper-scaling tank with serious threat from his passive in teamfights. Yet his passive has to be the worst scaling ability in LoL for one reason: the health drain scales harshly with level.
Each tick of drain is proportionate to 1+Sion's level. This means that the drain is 9.5x more intense at level 18 than at level 1. Sion starts the game at around 550 health, so to keep the drain proportionate, he'd need to have well over 5000 health end game. That's an absurd amount of health even with a pure tank build and healthy w farm, unless you build 0 resistances. So you'll bleed out faster late game than early game.
Then to make matters worse you have 0 scaling of the dmg or attack speed into late game, so anyone who buys armor will take minimal dmg even if you somehow get to them and stick. The net result is that Sion's passive is amazing early game and can easily net multiple kills in a lvl 1 or 2 fight, or steal the enemy's red/blue buff camp. But late game, when Sion is supposed to be at his strongest, the passive dies in a matter of seconds even with no enemy action whatsoever.
Therefore I propose Riot change the passive scaling to a % of base health (not sure exactly how much, but enough so that it's a nerf early game and buff late game), so that the passive has consistent strength across levels. This brings the drain scaling to about 3.5x as much level 18 as lvl 1. This way cheesy early game strats like the buff steal are less effective, but the passive feels less frustrating late game and forces the enemy to pay some attention to your lumbering corpse.
's unbreakable will. Adds some skill to the passive and fits thematically. And still counterplayable with a second cc or timing.
he has a really good passive because he is a squishy short range mage who even building RoA is not going to be tanky enough to survive if he goes in, however in return death only makes him stronger by taking away mana costs and making him immune to cc so that dying at the right time and place actually makes you really strong.
Nobody likes 
and 
passives because they are champs who should theoretically never die, and so when you do the passive just feels like compensation that would've been better spent keeping you alive in the first place. 
passive and 
passive are both decent but kind of feel silly because Anvia's passive literally just gives you a bit of extra safety from tower dives early game, and Aatrox's passive is just a GA on a shorter cooldown.
are exist. There is no reason to pick a late game tank who get melted by 
under few seconds or just get destroyed by 
and outsustained by that awesome 
) or in the form of these stupidly broken juggernaut things (
or 
will carve through you like butter. Not to mention without mitigation an adc will lifesteal tons of health from autoing you so they'll be full health when you try punching them to death.
