@Sibersk:
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Yes Luffy should be present for that, are you insane?
He doesn't have to be present for that when he can find out later. Similar to when Ace got defeated by Blackbeard. Luffy is not needed for a scene like that to occur. He only needs to react to it in some place and some time for it to feed into his motivations.
Assume the audience aren’t Luffy junkies
Use examples where Luffy is still there.
Because Oda is the Luffy junkie. Not the fanbase. Oda only keeps Luffy in the focus because he wants him in the focus. Not because it is unquestionably impossible for an arc to function and progress One Piece's overall story without Luffy. If Luffy goes to the Reverie, the main reason behind that is because he and he alone is not comfortable with having any arcs without Luffy in them. Not evidence that an arc can operate without Luffy and progress the main story.
And I used those examples because they clearly show that One Piece arcs do not need the whole Straw Hat crew to be present to tell compelling stories that move the plot along. So if that is possible, why is it such a stretch to have a short arc without Luffy present as well?
Half the time in almost every single arc, Luffy is stuck in some random place while the plot has to be supported by the other Straw Hats and supporting characters being more involved in intricate plotlines. The other half of the time, Luffy finally shows up near the end to help take down the main arc antagonist and inspire everybody. Throw in a couple in-between scenes where Luffy loses to the villain and makes friends with somebody on the island, that is pretty much the gist of what Luffy contributes to each arc like clockwork.
Luffy doesn't even know most of the tragic flashbacks or circumstances behind a lot of the arcs and takes down the bad guy purely because they hurt people he cares about. Luffy's main function in this story is to act as a pirate-y hero. And not every arc needs Luffy to beat up the bad guy and save the day. Like when Zunisha knocked Jack away from Zou. All Luffy did that arc was listen to flashbacks and party. And back on Jaya, he didn't even kick Bellamy's ass. He did it purely out of vengeance to get back a friend's gold.
If Luffy’s there then the story’s progressing. Not hard to see there.
What isn't hard to see is that you're stipulating an imaginary rule. As if no matter what happens, even if the Warlord system gets abolished, if Shirahoshi's identity as Poseidon gets publicly revealed, if a Yonko invades, if Ryokugyu and/or Vegapunk gets revealed, if Vivi and the other royals decide to pull some sort of rebellion, if the Revolutionaries show up and Sabo meets Stelly again, if Cobra gets imprisoned or killed for asking about Poneglyphs, etc., none of this progresses One Piece's story.
The story might have started with a singular focus on Luffy's quest to Pirate King, but it has become much bigger than that. Any long-running drama series is bound to get bigger than the protagonist's intentions. It's natural and those stories can shine the brightest when they capitalize on a network it gradually establishes to move the story to other places for brief periods. Any developments will all still feed into Luffy's endeavors somehow and it gives Oda more leeway to set up plot threads. This is nothing revolutionary.
And Luffy likely won’t be at the Reverie. But the Reverie will also not be a volume long or its own story. A revelation here, a set up there. Anything more important than that you can deliver by News Coo like the Revolutionary Base getting attacked.
The Reverie did not get built up for about two decades to be the typical in-between arc phase that only lasts two-three chapters. Especially when an editor literally name dropped "Reverie arc" and Oda said that Wano Country would likely start in 1.5-2 years in the summer of 2017. His predictions always end up going even longer than he intended, never shorter. And Whole Cake Island looks like it will wrap up by April.
You're only saying that Luffy needs to be present for the arc to progress or that the Reverie won't be any big deal only because Oda hasn't deviated from that formula too strongly. As if Oda can't change or that these types of scenarios haven't worked in many other stories. One Piece can be very formulaic at times, but Oda does like to gradually change up his game and save special events that completely defy expectations for rare occasions. This can definitely be one of them.