Sion is already so easy to outplay, and then comes his passive which takes a while to pop (everyone's already away from you by that time). Easily kited, weak speed boost(which decays too fast) it makes the most immobile of champs look mobile, HP drains too fast for someone that can easily be cced to death. in a game where mobility is taking over, i believe his passive needs a buff.
alfavhunter2 votesAdd onto Sion ult
Sion passively gains 33/66/100% slow resistance when in his passive
That way at rank 3 ult they have to use hard CC on you
Strawberrycocoa2 votesI've suggested this to people before, and I really liked the counter argument to it.
Sion's ult is easily avoided in lane phase, this is true. But teamfights are another matter. A Sion who dies in the middle of an enemy team can use his passive to charge right for the Marksmen. Suddenly people are scattering to avoid him, or focusing fire to kill him. That's something his allies should be following up on. A Sion who charges in CCs everyone then hounds them post mortem is providing a strong disruptive presence that his allies need to make use of, instead of running away once the tank dies like idiots.
Sion is a tank, he can draw cooldowns and eat enemy damage. His allies should make full use of the vulnerability windows he is creating by making the enemy team focus on kiting/killing his passive.
I also have found mobility items are very helpful for his passive's chasing. 
for example.
Maple Crusader1 votesI agree, make him run faster. Perhaps make it that any CC that Sion gets hit with, also delay his self hp drain.
Hot Belly Dancer1 votesI agree, make him run faster. Perhaps make it that any CC that Sion gets hit with, also delay his self hp drain.
or make him immune to cc during that time.
IcyPepper1 votesOr maybe just soft cc?
Make him immune or resistant to slows and roots? For the enemy to use stuns and such (which could be important in a teamfight, since wasting hard cc on the frontline can give an opening for an assassin to dive in).
Hot Belly Dancer1 votesi believe immune to cc and run faster would be great, as his hp gets drained really fast.
Hellpyre1 votesJust use it to push towers.
Hot Belly Dancer1 votesJust use it to push towers.
lol, what about teamfights then?
Hellpyre1 votesLeave the team, go push top.
Ralanr1 votesWish it was a shorter delay. Or have a faster movement speed or attack speed.