My crazy theory is that the Borg Queen turns out to be Beverly.
I think it'll be someone we know. Matalas has said we've only seen the back of the head so far. So, it seems to be setting up a reveal. And some of the voices Jack heard sounded like Beverly.
Those are in story reasons but there's also out of story reasons, mainly Jack's culpability. As it stands, you can blame Jack for all deaths and damage that occur due to the assimilation of Starfleet. He made it possible by running to the Borg queen. In contrast, JL was abducted and assimilated, so he gets a pass. But Jack ran to the Queen, making it possible.
Now, we know from interviews that the story ends in a happy place. That doesn't seem possible if Jack either is guilty of causing the assimilation or dies to fix his mistake. But if he ran to the Queen because he thought she was his mother, it gives him an out somewhat.
Of course, the big question is, if the Queen is Beverly, how has she been on the Titan and now Ent-D? I'd guess some weird combination of a Changeling controlled by a portion of the Queen still under Beverly's control. But I don't know why she wouldn't have her doppelganger tell Jack et al what was going on.
So, my brain is probably overthinking this, but I suspect some sort of weird revelation when we see the Queen's face!