Sion needs a buff, here are my suggestions
Playing Sion in the past several matchs has been extremely underwhelming. Personally, I consider Sion to be a "juggernaut" type champion, but due to his nerfs between his rework and the other top-laner reworks, he lost his luster. Sion is honestly hard to play in the top lane versus many other traditional top-laners. Many of these champions have movement speed built into their kit (Poppy, Fiora, GP are a few) and nearly every champion can out-damage Sion in a fight. If Sion pushes towards an enemy tower, it's a long, slow walk back to safety if a duel breaks out. I'm not saying Sion should be able to out-damage these champions, but when comparing Sion's damage compared to other juggernauts, there's not much competition. Overall, Sion needs the most help late game. Something needs to be done to make him less of a complete meat shield, and more of a threat.
With that being said I have a few suggestions:
Scenario 1: replace Sion's stats to around what they were originally after his rework. This will give Sion back his damage and make him more a of a threat in a duel.
Scenario 2: Add a movement speed bonus to Sion's Soul Furnace and either increase the shield amount or increase the damage. Sion needs something to make him less kitable. Yes, his ultimate is great for closing gaps, but it's extremely easy to dodge at medium to far ranges. Adding more damage to his shield brings back some aspects of the old Sion.
Scenario 3: Change nothing about his stats and make Decimating Smash uninterruptible by many other champion abilities. Certain abilities should stop Sion from casting Decimating Smash altogether (poppy ulti, Gnar ulti, ect.). Other abilities should not cancel decimating smash altogether; it should force Sion to cast it earlier or stall it's cast time (Bard ulti, Zillean double bomb, ect.)
Scenario 4: Change Decimating smash to if the spell is canceled early by an opponent, the cooldown for recast is lower & the cast speed is slightly increased.
Hope this feedback helps you in making Sion more competitive again.
in comparison to a lot of other tanks.
He can now splitpush.
if Champion.GG is anything to go by.