I think one of the bigger themes, if not the main theme, of the Elbaph Arc is probably "The Ghosts of the Past Have Returned to Haunt the Present".
I mean, you could almost say that about most One Piece arcs. A lot of these arcs are about righting wrongs from the past. But, I think this arc in particular is focused on that in a literal way moreso than others. Especially with all these Mythical Zoan Fruits acting as our "ghosts".
We're talking about Elbaph, the land that originated the concept of War, supposedly. It kind of makes sense that something about this island ties it directly to important events from the past.
We're learning a lot about the history of this world in this arc. And...it's all "forgotten" history, in different ways. We got the God Valley flashback and Loki's backstory, with Loki starting out being locked up and ignored by the rest of Elbaph. Now, we're getting Brook's flashback and learning more about Gunko/Shuri, who Brook thought he'd never see again. And we got the Mural and the Harley Text, which gave us a pretty important look into the bigger picture of the past of this World, which is all part of a forgotten history. Even Imu and Ragnir themselves are kind of living historical artifacts, that most people don't even know exist, both kind of hiding out like hermits from the larger world. All of this kind of hints at what we'll eventually see with the Void Century, down the line. History that has been "erased" coming back to dramatically effect the present.
Even the Nightmares kind of tie into the idea of the past returning. Because, it turns out, all these things the Giant Kids were afraid of? Most of them are actually real, in one form or another! Nika, Nidhogg, and the others probably all existed in the past. And now, here they are, in the present. Things that the Elbpah adults thought were just myth and legend...turn out to have some historical basis.
And the thing is, these "ghosts" aren't really "haunting" our main cast. The Ghosts aren't here to attack the Strawhats. The Ghosts are here for revenge...on Imu.
This isn't Usopp's arc. This isn't even Brook's arc.
This is Imu's arc. All the backstories that we've been seeing during this arc? They've been showing us times that Imu has tried to interfere with the World...and how it's all backfired on him.
During God Valley, we saw how that event not only helped to establish Roger as a force in the World, but it also led to Dragon creating the Revolutionaries, and Garp learning to hate the Celestial Dragons, and it turns out it's Blackbeard's origin story. During Loki's backstory, we saw that trying to control Harald just led to Loki hating the World Government. We just learned that Ragnir has a grudge against Imu and has been trying to find someone to help him defeat Imu. Sensing Joyboy's Haki Knot is what led to Imu getting more involved in current events. And Luffy's here with the Nika Fruit, which seems to be another old enemy of Imu. Even with Zaza, we're probably getting a glimpse at another character that has a grudge against Imu, if the Celestial Dragons fearing her is any hint. And chances are, during Brook's flashback, we'll probably see the first time Imu possessed Shuri's body. And, chances are, this will all lead to even Gunko/Shuri turning on Imu.
We've seen that Imu makes enemies everywhere he goes. He's kind of the opposite of Luffy in that way. And now it's all coming to a head here, on Elbaph. We aren't seeing the Strawhat's Nightmares come to life. All this is Imu's Nightmares coming to life, to attack him all at once. The consequences of Imu's choices are all coming back to haunt him.
Honestly? I'd ALMOST say it's possible we could actually get the Void Century flashback here on Elbaph, too. Because that probably plays a pretty important role in why this is all happening, too. Both for plot reasons and broader thematic reasons. And we are seeing brief flashes from the Void Century. But, I'd say we probably won't get a full flashback right now, because I feel like Oda's saving that for closer to the end. I feel like Oda wants to have at least one more big twist reveal, when he does the Void Century flashback, and he's probably keeping that for when we get to Laugh Tale or at least Lodestar, if they aren't basically the same place. But, we'll see. I didn't think we'd get Brook's expanded backstory this early, either. So, maybe a partial Void Century flashback could be on the table?
So, in a way, this arc is about your fears coming to life. But, not necessarily the Strawhat's fears. It seems to me like it's all been about Imu's fears, this whole time. We'll have to see how the arc ends. There might be another twist coming. But, for right now, I think that's what Oda is going for, and focusing on, in a roundabout way.
