Sion's passive predicament.

While Sion's passive can be scary and strong (good for him, yay), it doesn't proc off many item passives that would otherwise make him far more threatening and not so easily kited for 2 seconds (as more often tends to be the case).

Furor and Trinity Force don't proc during his passive, meaning what would normally address the incredibly easy counter of slows is no longer capable of doing its job (boo). This puts Sion in a place of basically just being the boogeyman for the couple of seconds that he has before he's rendered useless either by the distance the enemy has put between him and them or a slow or two on him instead (which renders this part of him clear countered by Rylai's and Frozen Mallet, as well as basically half the champion pool).

Ravenous also doesn't proc on his attacks, though this is a little more reasonable due to the fact that the potential damage output during this period would allow Sion to shred entire teams if they were locked down.

In all honesty, I really thing that just allowing the passives that enable him to be more sticky should still proc just to make him a little less laughable and a lot more of an actual threat rather than the giant scarecrow he currently is (because that's Fiddles' job). It'd be as simple as allowing boots or Trin Force to remain activated during the passive (I noticed that actives are greyed out due to being unusable, and it'd be simple to just create exceptions).