If the "Aiding Scion" is a fixed character (rather than randomly decided by a mission during the game, for instance), it seems too massive and muscular to be Sanction, on the cover we know.
Christopher has revealed the traitorous scion's character card will have mechanics that reflect it's duplicitous nature, so I doubt it will be randomly decided.
He also mentioned that the traitor is the only scion powerful enough to resist Oblivaeon's control. Even with what little we know about Big O, that's definitely not a feat to be taken lightly.
hmmm this begs the question is this someone we already know? Progeny maybe?
It is almost assured to be someone we know already, because there would be no point in hiding their identity otherwise.
It will be someone we know. From the livestream:
Mirado28: @gtgchristopher if we guess who the Scion is that's "aiding" the heroes will you confirm it or just deflect it in your usual wonderfully cryptic manner? 
GTGChristopher: @Mirado28 I will definitely not confirm it. You can guess all you like, but you will find out when you get the game in hand. Even the playtesters will not know.
cmschex: oh this is going to be a fun challenge in playtesting…
cmschex: trying to guess who the double agent is
GTGChristopher: @Cmschex Oh, you'll know the double agent. Just not their true identity.
cmschex: well that isn't mysterious at all 
GTGChristopher: @Cmschex *juggles smoke bombs*
That certainly sounds like it's a familiar face.
My guess is that the Cursed Acolyte is a Svion and OblivAeon took his pants.
It's a Fixed Point.
I wonder if Ra is a possibility?
Technically the first apperance of Voidsoul would be base Writhe's incap art from a timeline perspective. But good call on the flavor text of C&E being from Freedom Five Annual #30
Also! From the recent livestream, it was confirmed that Dark Visionary partly resides in the Void. The incap art for Citizen Hammer shows them in the Void as well. This seems rather significant to me. I am fully on the Dark Visionary = Scions Aid bandwagon after that little nugget of knowledge.
What about The Scholar? This is a complete shot in the dark, and I don't really have any evidence to back it up, except that we see him begin to vanish on his Infinite incap side, and this could be how he returns. That's a huge stretch but I've got no other ideas. Though I'd also enjoy seeing Dark Visionary as the aide as well.
My guess is Mrs. Parsons.
Idk about that really. One of the things I have been tackling with is the timeline of things. Let's examine:
Oblivion Shards are the direct result of OblivAeon being hurt (as we learned from the art streams). We also know from those streams that the Scion who is aiding us has hurt OblivAeon before (which was also hinted at in the Scions Aid comic). We have seen or heard of at least 2 oblivion shards in the card game prior to the final expansion (also confirmed by the art stream). Those 2 being the one Captain Cosmic/Infinitor found for their powers, and the one the Scholar found to create his Philosophers stone (and Scholars first apperance is Mystery Comics #1. Super early!).
Whoever the Scion is that is aiding us has been around for quite a long time and predates Scholar at the very least. What interests me about the Dark Visionary angle is the nugget about her being connected and partially residing in the Void. It's possible that through her interactions with the timestream and the Void being at least partially temporal in nature that the connection ends there. But DVis hits a lot of check marks for the Scion however. She has personal familiarity with a lot of our heroes, doesn't nessesarily wanna destroy the whole world, desires power, ect, and this Void connection is a strong link to the end of the story.
The shard that The Scholar found was created by OblivAeon being hurt, but it need not have been the mysterious traitorous Scion that did the hurting. I assume there are other entities in the universe that haven't visited earth.
OblivAeon being hurt occurred toward the end of SotM storyline and Scholar has had his stone made from a shard long before OblivAeon was a threat. So timeline wise that doesn't make sense
OblivAeon was hurt by the Ra attack. How would the Scion know OblivAeon could be hurt (remember the Scion comes to our heroes before Ra's big attack)? And where did the shard that Cap Cosmic find come from?
OblivAeon has been hurt before the Sentinel Comics timeline starts. Ras attack was not the first time he was wounded.
I said that in one of the art Q&As that it was said that the Scion is the one who did the hurting
We assume he has. I haven't read anything that says the OblivAeon shards came to be because OblivAeon got hurt. It could just occur because of his presence or essence that exists out there.
It was explicitly stated in one of the Art Q&As
Sorry, I missed that! I know it's been confirmed that the Scion had hurt OblivAeon, and that all Oblivion shards come from OblivAeon being hurt. Was it confirmed that the specific shard that The Scholar found came from the Scion hurting OblivAeon? (Or could it have been some other instance of OblivAeon being hurt?) Thanks!
Can you provide that quote because I definitely don't recall seeing that one.