I think Sansa’s plotting with Cersei to betray Dany (or pretend to), secure safety and power for what's left of her people and let Arya scratch that last name off her list.
Sophie Turner mentioned taking a scroll from the GOT set on Good Morning America and that it’s a huge spoiler. Assuming the scroll is mail come by raven, and not something Sam finds in WF archives (because why would anything there be relevant to Sansa's character?) where would RavenMail come from save King’s Landing, the Iron Islands (where Yara's headed or already arrived) or from Melissandre? KL is the only place that make sense for Sansa's storyline. That leads me to think there is some correspondence between Cersei and Sansa.
We know that WF is on fire and likely destroyed after Ep3, so where will the survivors go? South, to KL. Especially if they're going with Dany to claim her throne. Cersei's not going to let her enemies come into her land supporting a Targaryen queen with Dragons. So what if Sansa made an arrangement to grant Northerners safety/Independence for the North (maybe even the Riverlands if WF is no more) in exchange for Dany?
Here are the things in S8E1 that lend to this theory for me:
Arya/Jon reunion — “she’s the smartest person I know. We’re fighting for this family. Remember that you’re family.” If we're looking at it through a lens of Sansa playing the game of thrones and Arya being her right-hand man, it starts to look like Arya giving Jon a subtle warning. I think she's basically saying “Sansa made a deal to save this family, which includes you so don’t be mad later when you find out.”
Tyrion/Sansa reunion — This scene really just establishes that Sansa's smarter than she was and knows more about Cersei at this point than Tyrion does.
Sansa's posturing/dislike of Dany -- If she's setting Dany up or pretending to for the sake of Cersei and Qyburn's spies, why even pretend to like her?
The news about the Glovers just reinforces that she's the Lady of WF and reads all the mail
I think Cersei being happy that the Wall came down is about her plans being put into motion
We also have an exchange about Cersei between Sansa and Dany in the trailer for Ep2.
Cersei now knows Sansa didn't kill Joffrey, so maybe she'd be open to an arrangement. I'm sure she plans to betray Sansa and the Northerners but given Sansa's interest in Arya's kill list last season, I feel like Sansa's planning to let her little sis get her Needle on as payback for everything. (Which would be an interesting setup for CleganeBowl: Mountain goes after Arya and the Hound steps in -- but I digress.)
To what end though?
Maybe the Starks had a family chat before Jon got back and they all know Jon isn't their brother, but cousin and heir to the throne. Sansa's "you know I do," line could mean she's about to be a Kingmaker. She sells out Dany (or at the very least pretends to to Cersei who I'm sure has eyes in WF and Sansa's dislike of Dany is a ruse), kills Cersei, put Jon (and maybe Dany) on the throne and becomes the Lady of Winterfell, or Queen, or the Hand? Or maybe she has the throne tossed into the Blackwater Bay.
I don't know but I absolutely think Sansa is making moves in the background like Cersei and LF taught her.
This theory doesn't account for Bran, AotD, NK, the Lannister men or Jon's agency as a person lol, but poke holes in it! It's kinda crackpot but is any of this plausible to you guys?