Sion would be perfect for a Project skin

Now I know what you're all thinking:

"Project is a human only skin line. Sion is a zombie."

And let me tell you that you're not viewing the bigger picture. The project skin line in story is taking people and turning them into weapons of war/something not good to be utilized. That's what Project Ashe is leading her group to fight against.

They are basically an advanced version of Sion and this could be played into a Project skin with Sion being one of the earliest and most successful creations from the organization designing them. Forced to fight battles without realizing his free will is gone, but with tiny glitches in his programming that makes him unstable to control at times which is why he's not used often.

But then he's just Mecha Zero in thematics

Wrong again! Mecha Zero broke free from his design and seeks to be Megatron and control everything. Project Sion wouldn't have broken free, he (and several others) would be taken out of storage to fight the renegade Projects.

And there would be no saving Project Sion beyond killing him for good. He'd be too hard wired into loyalty for his faction (in lore, not in game) that even with his glitches he will never break free. Forcing the other projects to do the most merciful thing and kill him.

Tech magic shields and ults!

Now sadly this draws a lot of parallels to Mecha Sion. But thematically I can't think of a better champion that fits with the project group!