We’ve been told from the beginning (and even hinted as early as Marineford) that Ace spent a considerable amount of time in Wano. We’ve gotten hints of it through Tama and Yamato mentioned knowing Ace, but there’s still very little that we know about why Ace was in Wano and what he was doing there.
It seems a bit like a red herring (why would Oda decide to include Ace in an arc that already has so many characters with different roles to fulfill) and I think that this definitely needs to be expanded on during this arc. Initially I though that maybe Yamato was going to go into a flashback when she revealed that she knew Ace, but that didn’t happen. I don’t know when Yamato might mention more about her relationship with Ace, but I’m not sure how Ace’s history in Wano can be organically included into the Raid on Onigashima. My two best guesses would be that it could appear in a Kaido flashback or maybe in a reflection after the battle is over, but I personally think it makes more sense to happen after the Raid rather than during it.
4. Eustass Kid
Among the Worst Generation, Luffy, Law, and Kid have kind of been built up to be equals in a way. We’ve gotten plenty of exploration of Law’s character over the course of Dressrosa, but there’s still a lot that we don’t know about Kid. I’m not necessarily expecting us to get a long, detailed story about EVERY member of the Worst Generation (most of them probably will get some exploration but not all necessarily). But I think Kid with how he’s essentially built up to be a major rival to Luffy definitely deserves a lot of screen time that we haven’t seen yet. I’m not sure what form that will take in the story (maybe a flashback, maybe a big fight with one of Kaido’s crew) but I do feel that with his inclusion in the arc he will have a major role to play at some point. Law doesn’t necessarily need a major role in this arc since he already got plenty of time in Dressrosa so his role imo seems pretty satisfied as an architect of the plan and a major fighter. Kid still needs a little more time to flesh out what he’s going to do in this arc.
5. The Kurozumis
This point is definitely a lot more speculative admittedly, but like a lot of people I’m still not sure that Orochi and Kanjuro are dead yet. They very well could be, but it is certainly possible they could be alive thanks to their devil fruit abilities. I only feel like this point is worth mentioning because I could easily see Orochi acting incredibly bitter from Kaido’s betrayal and trying to do something in the shadows like kill Momonosuke. It could also offer an opportunity for Kanjuro to get some redemption by possibly stopping Orochi or helping the allies escape if the Raid fails.
Also, if the reveal of the traitor was supposed to be a huge deal in this arc it seems a little weird to me that Kanjuro’s death would be left offscreen the way it was. Deaths are rare in One Piece, and Oda usually likes to build up deaths to be these grand events (think Ace, Whitebeard, and Pedro). In fact the only time I can think of a character getting killed off screen off the top of my head is Absalom, and imo Kanjuro has more importance to the story than he ever did. Again, I wanna stress that they could very well be dead, and I would probably be fine with either case, but it makes more sense to me from a storytelling perspective that we see a proper conclusion to the Kurozumi part of this arc.
6. How will Luffy beat Kaido
Correct me if I’m wrong, but I don’t think it was that long ago when Oda mentioned that he doesn’t know how Luffy is going to defeat Kaido at this point. The way I see it, even if Luffy takes Kaido on in a giant team-up fight Luffy still needs to power up significantly to beat Kaido. I don’t think the Haki training he got in the prison is going to cut it. I think it will help in him out during a battle in the raid (and it already has with removing Yamato’s cuffs), but the power difference between Luffy and Kaido was so immense during their first encounter that I think Luffy still needs to undergo some serious growth before it’s possible. A major failure seems like it would make for great conditions for Luffy to go into serious training mode in Act 4. Let’s say Act 3 ends with the Raid failing, and Luffy falling out of commission. This might give them the time he needs to train in Act 4 while other characters (Zoro, Yamato, Kid, etc) have the spotlight before we can have a major comeback in Act 5 or something.