How To Make New Sion Viable
Sion is an undead JUGGERNAUT. His current identity is easily missed but highly rewarding super burst crowd control. However, when he misses those powerful spells, he is left out of place and fed to the enemy team to die. What he needs is increased innate tankiness to allow him to have a foot to put on the ground when the enemy dodges all of his spells and slightly more reliability, mainly on his ultimate.
They need to increase his health per level from +73 per level to +85 per level. This would increase to around 555 at Level 1 and his Level 18 health to around 2000 (this isn't rounding, this is literally about what his health would be). This would give him an appropriate and healthy amount of health while still being able to gain infinite health.
Sion's Q's charging pace needs to be changed. Instead of charging in a linear fashion, it should increase up to 66-70% the first second of charging and then the rest over the last second. Sion still needs to charge for a second to be able to deal optimal damage, but he still needs to charge for the full duration in order to pull off the extreme damage and crowd control the spell provides.
Finally, Sion's R needs to be modified. HE SHOULD NOT HAVE A SET SPEED HERE! What should happen during his ultimate is the movement speed cap is temporarily removed and his movement speed is increased to around the current amount (950). Then if he has boots of speed, it would increase to 975, then 995 for second tier boots, and then increased by a percentage with other movement speed items. Also, personal preference, but the AoE at the end of this move in my opinion needs to be increased by about 33%.
If all this happens, Sion would be a great champion and be able to actually hit his spells. If he misses, he would have a reliable health pool to retreat with.