Assuming that it’s 3 POVs like in the previous books, I think it would be Ghost, Rain and Dasher.
Ghost since there isn’t a reason for his POV to be left out and his final chapter in The Dark Sun is left at an interesting and potentially plot-changing cliffhanger, Rain since Brawnshanks will be targeting her next so it stands to reason she’ll have a POV in that (plus I can’t see Rain not having a POV for 2 books in a row considering she’s one of the main characters), and Dasher since he’s on the cover and a “side character” being the third POV would follow the pattern from The Dark Sun with Nimbletail having a POV.
I don’t think Leaf will have a POV since her exiling herself is the perfect way for her to temporarily exit the story until the end of the fifth book/start of the sixth, plus if my theory is correct that the 3 dragon speakers will take turns not having a POV for books 4-6, Leaf’s turn to not have a POV will likely be next. I do think it’s possible for Nimbletail to be a POV character instead of Dasher considering how her storyline as Crookedclaw’s spy isn’t over yet, but at the same time if Nimbletail is staying with Rain then it wouldn’t make sense for them both to have a POV, and in the end it’s more likely for Rain to have a POV than Nimbletail.
I’m honestly not sure if I like the new approach of only 2 dragon speakers having a POV per book, I like seeing the siblings together more (not as in them being in the same place and working together, but as in all 3 of their POVs in the same book) and Rain being left out in The Dark Sun strips us of an opportunity to see things from her perspective, such as her relationship with Pebble and Lychee, how she isn’t as a dutiful dragon speaker as Leaf, and her reaction to Pebble’s death. I do understand why the Erins chose Nimbletail to have a POV in The Dark Sun instead though, she’s quite a compelling character and her plotline gives much-needed info/lore for the monkeys as well as letting the reader know what’s going on from Brawnshank’s side of the Bamboo Kingdom. Rain also didn’t really do anything that plot-relevant in The Dark Sun compared to Leaf and Ghost so all in all Nimbletail was a good choice imo, and although I would have liked it better if all four of them had a POV (similar to how 4 characters had POVs in the same book in Omen of the Stars from Warriors) it would likely be a lot to juggle in a book of this length without it seeming rushed. Still, I feel like one of the strengths of Bamboo Kingdom as a series are the three protagonists and how each of their stories are equally interesting and important, and having each book from books 4-6 lack one of the protagonists takes that away a little. TL;DR: I have mixed feelings about this, while I feel like it’s a necessary change and can better develop side characters, I also prefer how it worked in books 1-3.
Orrrrr I could be wrong about all this and all three dragon speakers will be protagonists in The Lightning Path and the sixth book. 😄