I have a number of thoughts on the matter that I'm opting to put here rather than clutter up the first post. If Sion cannot be banned and is insanely strong, clearly blue team will have a sizable advantage, but perhaps not an insurmountable one.
AP Sion's strengths are waveclear, durability, zoning, and roaming. However, he is immobile, short-range, and struggles in teamfights where there's many foes who can wipe away his shield. My buffs do not change the fundamental calculus of Sion.
Sion's effects would be most immediately felt in mid lane. One team would almost certainly pick Sion, and the other team would have to choose someone who could survive lane against a hard-shoving, basically invulnerable, undead juggernaut. LeBlanc would immediately vanish from competitive. Her waveclear is dicey, so she'd struggle vs. Sion's ability to wipe the whole wave instantly from level one onwards. Though she could probably avoid dying against him, she couldn't puncture his shield without burning pretty much her full combo. She could try roaming, but Sion would be roaming too, and not missing out on farm. Melee mid laners also become unviable, as they can't even farm without getting killed by Sion. Imagine Yasuo trying to deal with a 300 shield Sion at level one!
Range and safety would be the key to surviving mid lane. Super long range mages like Ziggs, Xerath, and Lux would see a boom in play, because all three can farm whole waves without leaving the relative safety of tower. They also have CC to stop Sion from just running at them.
The effects of the Sion buffs wouldn't just be limited to mid lane, however. Bot laners would have to deal with strong waveclearers in mid lane, as well as the roaming from the game's Sion. I think Sivir would benefit perhaps the most from this. Her spell shield gives her great resistance to Sion's combo, her ult allows her to escape from an enemy Sion or allow an allied Sion to run in more easily, and her Q and W allow her to push just as hard as the push-heavy mid laners. Caitlyn and Corki with their range and safety would also become better picks. In terms of support, Zyra and Morgana's strong disengage would be enough to keep them viable picks, but Lulu and Leona probably would probably fall off. Janna might also see a resurgence.
Top lane would become much more cautious and tower-huggy. The threat of roam would be much greater than in the current meta. I don't know as much about top lane, so I can't really speculate on specific champions that would be helped or hurt by this.
Jungle would probably switch more towards farm-heavy junglers. Sion is pretty much impossible to gank, because he's so durable that he could walk away from even the best ganks. Top and bot would likely become more safe and defensive, so the avenues for a gank onto them would also be fewer. Shyvana and Nocturne would do well with their abilities to countergank, but even folks like Yi or Jax might start doing well in jungle.
Buffing one champion would have repercussions far beyond that one champ.
