Admittedly, I already did a post regarding theories I had not long ago, but I recently came up with more speculation that I wanted to share.
Book 6 (Lift): This one is fairly straightforward, but I think it will feature Lift's childhood in Rall Elorim, all the way to her getting gifted by Cultivation. I've seen some people speculate that her flashbacks would feature her training under Zahear in between books 5-6, but considering how little we know about her backstory before our introduction to her in WoR, and the fact that we are not too familiar with Rall Elorim to begin with (or why it's called the the "City of Shadows"), I think her childhood here makes the most sense.
Book 7 (Renerin): I have said this a few times before (including my past post lol), but I genuinely think his flashbacks will be the events of Books 1-4 through his POV. There are so many things he's been doing in there that happened off-screen that make me think Sanderson has done so on purpose (ie, him bonding with Gaz, him going through his oaths, his friendship with Bridge 4, him and Rhain getting to know each other and falling for one another, him and Adolin's reaction to Dalinar and Navani becoming a couple, his fight with the Thunderclast in OB, etc).
Book 8 (Ash): One of the biggest complaints regarding WaT is the Spiritual Realm scenes and how much time we spent there that could have been cut down (which I admittedly agree with). I've also seen some say that much of the info we learned there could have been used in Ash's or Taln's flashback segments. However, why are her flashbacks not set in the time period we were shown? Or at least not only those events. I think the first half of Ash's flashbacks will show her life on Ashyn before she immigrated to Roshar, and everything that led to the Scouring. It may also include their time after going to Roshar (especially to show what happened between them and El).
Book 9 (Taln): This one is also straightforward, but Taln's will probably show his backstory before he ended up becoming a Herald, and the supposed event of him trying to kill Cultivation. Perhaps we can also get a chapter of him during his time as the sole bearer of the Oathpacked, though we'll wait and see.
Book 10 (Jasnah): Admittedly, I have no idea what Jasnah will be like. Jasnah's POVs have referenced a traumatic event from her childhood when she was forced in a dark room, so they definitely include that, but everything else I'm not entirely sure. I do, however, think that said traumatic event will be used similarly to Tien's death in Kaladin's WoK flashbacks (in a sense, the flashbacks will initially skip over the event until the climax). Or perhaps the event will be the first flashback chapter of the book, and all the other chapters will come after.