Sion needs better stats.
And surprisingly, no, the problem is not mainly his HP. It's his base armor. It sucks, flat out, compared to what he needs to prioritize to be effective: health. Some scaling armor buffs would go a long way for Sion, since Cho'gath just took his place as the infinitely stacking HP tank. FFS when even Shen has better scaling armor, you know there's a major issue.
>Tl;dr: Read the fucking post it's a paragraph.