But if he was, there are only two things I would want done to him.
1. His passive. Please let Sion's boots actually mean something in his passive. Maybe it's just me, but I feel like Sion's speed is decreased when in his passive. So much that people can simply outrun you without using a dash or slow. This also makes Sion's MS boost feel less impactful (imo) because the speed increase decays too quickly for you to land even two hits on a target. If you don't want to change the MS then at least shorten the delay.
2. Stop letting people go through Sion when he's charging his Q. He's a zombie, not a ghost. They are completely different types of undead. I know it's a form of counterplay, but Sion already has a bunch of counterplay (his Q alone has _three_ forms of counterplay at least if you include this. Is there such a thing as too much counterplay?).
These are mainly personal issues I have with Sion. Overall he's very well balanced and doesn't really need changes. But if he got any, these are what I would prefer.