I agree with @Pagerunner's approach at choosing the 9 characters Brandon is referring to, though I wouldn't write off Lift... Who would be included at the expense of Renarin, Adolin, or Navani. Probably Navani in my opinion, of those three, but it's a toss up.
What do these ten characters probably have in store?
Kaladin - I don't think we have too many clues from Oathbringer concerning specific things Kaladin needs to do next aside from swearing the 4th Ideal, and that could happen in countless ways. But the Lirin PoV gives some big clues for what Brandon's planning I think, despite the fact that he has trashed it. With Alethkar occupied and the Coalition in control of Jah Keved, there's bound to be conflict in the region around their northern border. The Herdazian general is apparently involved with what's happening there... and so is Kaladin. It makes a lot of sense that Kaladin would have a team that's active in the area.
Shallan - I think we're going to see a lot of interaction between her and Adolin, her [proto-] squires, and her brothers. She and Pattern noticed that Shadesmar is "weird" in some way around urithiru, and I'd be surprised if she weren't involved in following up on that. This points at Shallan in Urithiru. I also think she's going to have more dealings with the Ghostbloods in book 4, and they've asked her to get in contact with Sja-anat. Not sure how she can do this while stuck in Urithiru... So maybe her brothers won't matter as much and she'll be headed out into the field. Hard to predict where Sja-anat will be next though.
Dalinar - Dalinar's role will apparently be downplayed in book 4, and that's not surprising. He'll be busy politicking and establishing his refounded Knights Radiant.
Venli - I see Kaladin mentioned as a pairing for her, but I have a hard time seeing how that would work out. I think Venli will primarily be among the singers, which isn't really a safe space for... any of the other characters. I feel pretty strongly about this though. I think her role in the story is to lead her people in a rebellion against Odium and the Fused. It's possible that she'll run off and then return to do this later... But I think it's more likely that she will have plenty to do where she's at. It's possible we'll see some battlefield interactions, but Venli's not much of a fighter. Part of me wonders if Moash could be one of the 9 characters, because I can definitely see her interacting with him. (and we do need to keep tabs on Moash) I've seen her mother and Rlain thrown out as pairings, but neither of these would be a main character, whether they interact or not.
Szeth - He swore his oath to go after the Shin, but that's certainly going to have to wait until book 5. He could be hanging around Dalinar, but if Dalinar asks him to do something he's going to go do that. He does probably need some setup for book 5, but who knows what form that takes beyond a bit of screen time. I don't see him having anything interesting to do while stuck with Dalinar in Urithiru. I think he'll be out with one of the other two groups.
Lift - Not much to go on with her either. Definitely an easy argument to make that she's mostly sidelined as a PoV character.
Renarin - We need to explore the nature of Renarin and Glys to some extent. On one hand, that could happen anywhere. On the other hand, who better to pair him with than the girl who's supposed to seek out Sja-anat? Shallan/Renarin road trip?
Jasnah - I've seen lots of people wishing to see her go off on some awesome adventure, but she's the queen of Alethkar now. I'm not convinced she has time or freedom to go skipping across Roshar, putting her life at risk. I put her firmly in Urithiru, doing politics and figuring out how to "fix" the city. She'll definitely make some Shadesmar trips in or around the city, so it won't just be meetings for her.
Adolin - Brandon has suggested that he hasn't escaped the consequences of killing Sadeas yet, so perhaps this will come back to play--and that likely means being in Urithiru. He's also a Highprince, which means he's on a short leash like Jasnah. We also need to see some Adolin-Shallan interaction, but I don't suppose Brandon will bend the story out of shape to keep them together. There's not a lot here requiring him to be a PoV character though.
Navani - Another Urithiru character, I assume. Her interest in fabrials makes her a likely candidate to help out with the city/Sibling mystery, and I don't see her going far from Dalinar. I don't see any reason for her to be a PoV character for this, however.
So how to put all of this together?
I have to agree that Lift seems least likely to be a main character of the ten listed. Navani, Adolin, and Renarin seem next most likely to be sidelined.
We've got several characters who are likely to hang out in Urithiru, so it's easy to imagine the 4-PoV primary arc will be here. Dalinar and Jasnah are definitely here in my opinion. Navani is also definitely here, though hard to feel certain about her position as a major PoV character. Shallan, Adolin, and Renarin could have things to do here. Szeth and Lift could be thrown into the pile. I've got a little bit of hesitancy to assume this is the primary arc though, because humans stuck in Urithiru doesn't sound particularly... dynamic and exciting. Great content... but the primary arc of the book? Hmmm...
Two options for Venli. She's either deep in singer territory, and probably interacting just with Moash (these two forming the 2-PoV tertiary arc)... OR she's closer to the front lines and somehow interacting with humans there.
Everything I've noted above under Kaladin DOES make me feel very strongly that he will be very busy in northwest Alethkar / northeast Jah Keved / Herdaz. I'm not entirely sure who will be sent with him, however. Adolin is a pretty good option. Makes plenty of sense that he'd be on the frontlines trying to take Alethkar back. And he's got a lot of chemistry with Kaladin. Szeth is another good option, given his fighting ability and the fact that he can keep up with Kaladin. Could toss Lift or Renarin here, I suppose.
I think it would also be good to step back from the characters for a moment and take in the bigger picture. I haven't mentioned anything concerning the western half of Roshar, which perhaps feels like a big oversight. Then again, as much as I want to see more of these nations, I wouldn't be surprised if our time there is limited yet again. The Dalinar (or others) could make a few minor field trips away from Urithiru if necessary. I can see a lot of information about the Makabak region coming second hand. (i.e., conversations and reports from the Azish) Everything more towards the northwest has largely been a mystery all along, so not surprising if we continue to get little from there for now. We've also got interludes to help keep us on top of these areas. So I don't see a big problem here.
However, I DO think we need to see the Horneater Peaks. These are very important to the defense of Jah Keved, and Oathbringer made it abundantly clear that something is happening there. It's been a whole year since the Battle of Thaylen Field. Our characters HAVE to handle... whatever's going on there, in the next book.
With all of this in mind, I've got two propositions:
Who: Venli, Kaladin, Adolin, and Szeth Where: frontlines the northern Jah Keved / Alethkar / Herdaz
Lots of direct human-singer conflict. Venli gets to see her people thrust in the middle of a somewhat reluctant war, and Kaladin is a good, empathetic person for her to run into here and there. Adolin-Kaladin chemistry will be on point. Szeth is maybe a bit out of place... I'm looking forward to the Kaladin-Moash confrontation(s).
Secondary Arc Who: Dalinar, Jasnah, Navani Where: Urithiru
Politics, Knights Radiant rebuilding, and the Urithiru restoration project.
Tertiary Arc Who: Shallan and Renarin Where: Horneater Peaks
Wild speculation here, but I like it so I'm sticking with it! Word is that that strange spren are starting to pop up around the Horneater Peaks, on top of the other weirdness going on there. Shallan and Renarin, escorted by Rock and some of Bridge Four, are sent to investigate.
Who: Dalinar, Jasnah, Shallan (and someone else... Adolin, Navani, Renarin, Szeth, Lift?) Where: Urithiru
Politics, Knights Radiant rebuilding, and the Urithiru restoration project. Not entirely sure how to connect Shallan with her Ghostblood activities while she's here, but presumably some of that.
Secondary Arc Who: Kaladin (and... others. Szeth, Adolin, Renarin, Lift?) Where: frontlines between Jah Keved and Alethkar, including Horneater Peaks
Area is maybe too broad to be reasonable, but it might work. The "who" here is a big toss up. In fact, it's possible that this is even swapped with the primary arc.
Who: Venli, Moash Where: singer territory
We're deep in singer territory, dealing with mostly internal singer issues.