Here we go again.
I'll split this into three parts for my sanity.
1. Nakama left behind
2. Oden's Will and the Ancient History of Wano
3. Pluton and other lose ends
1. Nakama Left Behind
There have been two instances of Nakama being promised seats on the crew before Wano; Vivi in Alabasta, and Jinbei in Fishman Island. Wano had... four. Yamato, Momo, Kinemon, and Tama (with a Ninjutsu necessity), were all given invites by Luffy.
Carrot is another story that can be addressed outside of Wano in multiple ways when it comes to joining, and you can argue Momo and Kinemon will never be able to travel due to their commitment to Wano, but, Tama and Yamato have DIRECT ties to Ace, and both wanted to sail out of Wano with him.
This series is about Inherited Will, and if Ace's Will was to sail with Yamato and Tama, and Luffy met and freed both of them, it stands to reason they will sail on the Sunny.
No other island has had close to this many people with direct invites to sail on the ship, who wanted to sail at the end of their arc, but couldn't for other reasons.
So, why was that, exactly? Why did Oda leave them all behind? Well, that's because, we never really finished Wano, did we?
2. Oden's WIll and the Ancient History of Wano
Oden's Will was never fully about Kaido. It was to open the borders of Wano. Oden knew of Joy Boy, and wanted the country to be ready to welcome Joy Boy (Luffy).
This is why Yamato couldn't leave; she wishes to fulfill the Will of Oden, and won't leave Wano until its borders are open. However, she ties much deeper into things.
Wano has a history of racism that is still unaddressed. The Kurozumi were persecuted heavily, and we now know from the SBS that Tama is a Kurozumi. The cycle of hatred and revenge hasn't been broken; once Yamato is well known as the child of Kaido, and Tama as a Kurozumi, they will be hunted.
This connects back to the Ancient History of Wano, as Kaido stated Oni/Ogres could never get along with humans.
The history of the Ancient Oni race is a mystery we have little info on so far. We know Vegapunk tried to clone Ancient Oni in Punk Hazard (the Numbers), and that Oni like Oars were gigantic. But, there's so many missing links between Oars, and the giant Oni skull that makes up Onigashima.
Yamato is the key to unlocking that history of persecution against Oni. I believe that Onigashima is Joy Boy's skull, and that Joy Boy died holding up Wano, which is why the country looks the way it does. Either way, the history of the Oni race and Yamato is clearly important to the narrative, just like the extinction of Dragons and the Celestial Dragons obsession with them.
This isn't even addressing the lack of Ryuma lore and Ancient History lore, and Toki's backstory.
3. Pluton and other lose ends
Which brings us to Pluton. Oden's Will was to open Wano. How does that happen? Well, not only does Wano lose its greatest defense, the Borders Opening also unleashes the Ancient Weapon Pluton.
Pluton has been the focus of two different arcs, and mentioned all over the place in the story. Crocodile and CP9, two main villains of the Straw Hats, both fought for the sake of reaching Pluton. Oda isn't going to offscreen the Ancient Weapon he's spent the most time on in the story.
Pluton's presence is a giant Chevok's gun, and guess who knows about it? Yep, Caribou. The Swamp Guy. And guess where he is? Egghead Island. And guess who has a ship sailing to Egghead Island? The Blackbeard Pirates.
When we ended Wano, Luffy gave Momo the Straw Hat's flag, and told him to hang it high, and point to it if they ever got attacked.
Blackbeard's forces are all near Wano, and Caribou is right next to some Blackbeard Pirates to tell them where Pluton is.
So, post Egghead will be the Straw Hats protecting Wano as multiple forces attack it, with Blackbeard trying to take Pluton the focus.
This will allow Dr. Vegapunk to use his knowledge of artificial Devil Fruits to heal the SMILE users, the Straw Hats to reunite with Yamato, Momo and the rest, Zoro will get his backstory, Kaido and Big Mom will have their defeats addressed in some way, and some other crazy shit will happen.
Oda loves Oden so fucking much, there's no way he's going to offscreen Oden's dream, the Borders Opening.
People just hate Wano and don't want to go back to it.
But, we will.
Egghead will get ripped to shreds, S-Bear will do a Kuma teleport, and we'll get to Act 4 of Wano for Pluton and the Borders Opening.
People can hate it as much as they want, Oda doesn't abandon big plot points like that. The series is all about character's Will and Dreams. We've all seen how Oda wanks Oden, good luck convincing me he's going to offscreen the ultimate Oden wank.
