Calling it now, Jace, the Theorist is actually Kasmina in disguise.
"His hand on her shoulder. Reassurance flowing from his mind to hers. "No one ever appreciates the person who makes the hard choices. We won't be remembered for what we're doing, because in the end, no one will know it was even done. But we didn't do any of this to be remembered.""
Kind of a hail Mary throw this one is, but I think the 'Mage in Blue' in the prophecy, is actually Kasmina. She is currently disguised as Jace, taking advantage of the Reality Fracture that he had caused in his failed attempt to control the Meditation Realm.
(I think Jace is currently still reforming in the Meditation Realm. Otherwise, it's strange that Jace isn't presented in a spirit state like Ugin was when he reformed.)
In addition, quite a bit of Kasmina's kit is similar to Jace's own, but instead of outright mind domination like how he does it, Kasmina is more personal and manipulative by getting close and touchy. She's fond of collecting Planeswalkers and Embers (Non-Ignited Planeswalkers) as part of her shadow cabal, planning for a future encounter.
Also calling this now as well, [[Tam, Observant Sequencer]] is one of Kasmina's owls who has been polymorphed into Tam. (Also note, the set symbol for Secrets of Strixhaven is an Owl face. A familiar that's associated with Kasmina.)
Finally, despite Strixhaven being Kasmina's base of operations, she was wholly unmentioned in the entire SOS story outside of two back-to-back sentences who offhandedly mentioned her.