Sion's Ult and Passive.
In my opinion, and this is after playing many games with Sion, his ult, and passive, are pretty bad. His ult, good damage, nice long range, HORRIBLE MANEUVERABILITY. And I mean horrible. Everyone sidesteps you. Like, if you're going to make his ult this hard to turn, at least make the hit box big, to not only compensate for the bad turning direction, but as well as the loud charge, which pretty much makes everyone spread out at the same time. And, make it slow heavily, instead of it stunning sometimes. If it was a heavy slow, and a stun, that'd be pretty broken.
His passive? Give him lots of health, but reduced resistances. Make him go faster, at the bare least. You'll never kill anyone with his passive, unless they're not paying attention.