NickToons’s Book 3 trailer included some footage from Book 4, particularly ones of Kuvira. It must be another oopsie on Nick’s behalf because there’s no reason to include unaired Book 4 footage in a trailer for Book 3.
Kuvira’s importance has been foreshadowed in the finale of Book 3 (and even earlier than that), and since then, people are speculating her role in the next book. Some even speculate that she’s Suyin’s & Zaheer’s lovechild, but I personally think it’s too far-fetched. I do believe that there’s more to the story regarding Zaofu’s and the metalbenders’ ties with the Red Lotus.
Here are two shots of the unseen Book 4 footage from the Book 3 trailer:
Here we can see people from the Earth Kingdom being arrested? One possibility is that Kuvira is training a bunch of earthebenders to learn metalbending to free themselves from the handcuffs, but Kuvira looks pretty devious and the handcuffs look platinum.
And here’s another shot of Kuvira coming at the camera looking pretty sadistic. But that’s just my interpretation.
Her outfit is completely different from her outfit in Book 3.
Overall, based on these two shots, it’s hard to tell but one can lean towards saying that Kuvira is not a protagonist.
But can we just talk about how the screw-ups from Nick never end? Honestly I’m not surprised. But hey, at least the fandom gets more to speculate and theorize about during the wait for Book 4.