Here is a variant of the ancient evil idea that just occurred to me. It's my new favorite leading idea.
Consider the Borg Queen. What is she? We've seen several and know there have been many. When one dies, a drone becomes the Queen. We can say it's a "personality program" which can be grafted onto different individuals (the FC Borg Queen, the Dark Frontier queen who was a different species, Jurati) to fulfil the need of the collective to have a moderator who brings "order to chaos".
But why would that necessarily be the only personality program? We saw them individualize Seven of Nine (hold that thought). We know the Borg can make individuals as needed. Who is to say the Queen is the only personality program they have stored?
Similarly maybe Locutus isn't an ancient evil in the sense of something the Borg assimilated a long time ago that's now free of it's Borg shackles. Maybe Locutus is also "personality program" they load onto a representative assimilate particularly resistant species, and that every time they succeed, Locutus grows in experience to apply forward to the next species it is used on. If the Queen is the program loaded to moderate the collective, Locutus is the conqueror loaded to ensure the collective grows. Maybe Picard was like the 423rd Locutus host and had the invasion succeeded, Picard-of-Borg would have been decommissioned and the Locutus personality program idle until the Borg needed to deploy it again.
Under this theory, it's manifesting in Jack now, because it was (unintentionally) passed down biologically (remember: Borg are biological and technological) giving the Locutus personality program - mostly trapped inside Picard's corpse' parietal lobe - a way out of an immobile corpse and back to the collective. Vadic was going to load the Locutus personality program she harvested from Picard's corpse onto the ready-made host in Jack, which would be familiar to Seven who had her own personality loaded back onto her body when the Borg chose her as a representative, and would be familiar with the nature of the Queen. This is what Vadic was referring to when she said to Seven that it's "good she witness this".
In summary:
-Locutus is another "program" the Borg keep making versions of like the Borg Queen, and it is ancient in that they just keep making new ones and passing the knowledge down to the next version.
-Picard wasn't the first host for the Locutus personality and wouldn't have been the last.
-It's trapped in Picard's corpse parietal lobe, communicating by manipulating nanoprobes in Vadic, and looking to get out. It "awoke" when Picard moved his mind to a synth golem.
-Jack was born accidentally as a ready made host for Locutus since the Borg are also biological and aspects of Locutus were passed down in Picard's DNA. Once "awake", Locutus sense him and wants to merge with him.
-That's what Vadic was about to do on the Bridge of the Titan and wanted Seven to see it because Seven would understand that Borg individiduals are loaded programs (basically) and that Locutus was just a misplaced one.
-The face of the antagonist in the last two hours will be Locutus with Jack's face (and not some new actor or a CG creation), and the plot will revolve around stopping Locutus without killing Jack. A father/mother save their son from demonic possession story, but with Starships.