How to balance Sion
As a diamond player on my main account (Clapp dat booty) and sion/poppy main, I know what his greatest issue is currently and why Sion is too strong.
After he got his W and ult buffs, they were to supplement the nerfs they did to sunfire cape where it only did 50% of its current damage to champions but 200% bonus to minions. Ever since the revert of sunfire cape, his old buffs caused him to way overtuned but I do not think sunfire is the issue. Champions like garen really needed that sunfire revert to remain relevant.
As a sion main since his rework, I love him to bits but I know how strong he feels comparably to when I played him in the past.
His Q (decimating smash) is in a very good position currently and doesn't need anything done. His W (Soul furnace) is good currently as well as his ultimate though maybe could be adjusted (trying to be unbiased). Maybe have no-charge-up/point blank damage on his ult be slightly lower to incentivise him to use at great distance or purely use close as CC. Perhaps have his W shield include slightly lower HP ratio. Maybe even slightly lower is HP scaling per level as he gets HP scaling based on his W passive (though Sion support would be hurt). However I do not think these abilities is where his primary issue is currently and these don't need to be changed IMO.
Sion's greatest issue currently is definitely his E ability (roar of the slayer)! It is way too safe for someone that should be required to get into close ranges with minions and champions during laning phase. Not only is it an amazing wave clear and management, but it is also a huge poke that will wither down the opposing laner. I think it is a good ability by design as it allows Sion to still farm in poor matchups but it is way overtuned and should not be a go-to max first ability in EVER matchup! Ways I suggest to adjust it would be lowering its base or bonus damages. Like maybe max rank 5 it only does 170 dmg versus the 210 but increase its AP ratio to 50% from 40% so AP sion isn't shafted. Or maybe have the ability do even less max rank (140 damage) but have it do an additional 50% dmg to minions so the numbers are the same for minions but the poke it less harsh and therefore primarily a farming tool and a teamfight debuff. The last suggestion, and probably the best imo, is to remove the bonus 30% damage as the launched minion does additional damage to other targets hit. Using its great bonus range and multi-hit nature should be good enough incentive to push minions into your enemies and the bonus damage just adds up too harshly as non-interactive poke and farms waves way too easily. Plus this would be almost unnoticeable in teamfights as you can rarely find opportunities to push minions or summons into enemies (though sieging late maybe a bit trickier).
If the removal of the bonus damage from pushed minions is too harsh, it can be adjusted to only do an additional 10% or maybe just increase base damage to 240-250 from 210 max rank while removing the 30% bonus damage. But I doubt it will be too harsh as he has been a nuisance for a lot of people and I am tired of seeing my main banned and I know how strong he feels currently where novice players are picking him up and still being very effective. Sion has always been about timing the W shield to absorb as much damage as possible and still dish out its explode/diffuse dmg and positioning and timing your Q and R wonderfully to maximize his CC and damage. His E ability is just too strong for something that is very easy ability/safe in his kit and therefore can be abused. I hope someone takes notice and maybe a Rioter will take notice and hire me for balance team xD I got plenty of other great ideas.
Or maybe discussion why Sion is too strong and hear other people's ideas or suggestions.Or maybe just keep him the way he is a leave my man baby un-nerfed. I'd still be happy if unnerfed as I love sion but again I know how strong he feels compared to the past and figured I'd give good insight.