My probably wrong theory is that Jack is possessed by a Pah-wraith and the rebel Changelings wanted him because with Sisko Prophet protecting the wormhole, they’re cut off from the Great Link, and who better to help than the Pah-wraiths?
I hope not.
I hope Jack's visions/capabilities don't have anything to do with Prophets/Pah-wraiths.
BUT, Picard has a Reckoning Tablet in his study.

Prophets/Pah-wraiths, Borg, candle ghosts, an aftereffect of the device from "Attached"? Nothing that I can think of would be a good resolution.
But the worst thing is the "Irumodic Syndrome" angle.
The "Irumodic Syndrome", the "overclocked brain", is Picard's and Jacks's "superpower".
The Picard that we knew from TNG was the way because he was "special", he had an "overclocked brain".
If the writers go that route it would be another character assassination.