Well, unfortunately, I don't think we're going to get a lot for at least a few of these guys.
Dragon and Akainu, I could honestly getting the least amount of backstory and action. I feel like we don't really need THAT much backstory for either of these guys. Ultimately, they're both pretty simple guys. Maybe one chapter of backstory each, since I don't think we need much more than that. And I could see both of their final battles (possibly against each other) being relegated to the B-plots of a few chapters and not really being the main focus of a entire arc.
With Shanks, I honestly don't think we need a terribly long backstory. We've already gotten a lot of his backstory via flashbacks with his time with Roger and Buggy. We really only need to know how he's related to Garling and what Roger told him when he got back from Laugh Tale. I could see Shank's backstory being wrapped up in 3 to 5 chapters. Action-wise...yeah, I could see there being a semi-lengthy arc with Shanks. Especially if Shanks and Luffy do end up fighting each other. But, if Oda goes the "Battle Royale" route, Shanks' action might be shared by fighting Luffy and Blackbeard and Buggy all at the same time, potentially.
Imu is kind of the reverse of Shanks. I could see Imu getting a pretty long backstory, if only because it'll probably ALSO be the story of the Void Century. But, I don't know if Imu's fights will be that long...or if they'll even end up fighting Imu directly or not. Blackbeard might swoop in and steal the show. Or stopping Uranos might be more of the focus than fighting Imu during the final arc. Or maybe we WILL get a long fight with Imu? It's a little hard to say.
Mihawk...is going to be a complete wildcard. Backstory-wise, his story could be really simple ("I wanna be the strongest swordsman in the world!") requiring only one or two chapters to get into. Or it could be a longer 5 to 10 chapter epic, like Kuma or Law's backstory. And action wise...I would really like it if we'd just take 5 to 10 chapters to really focus in on Zoro and Mihawk's fight. It's been built up enough, it could be a big epic moment. But...realistically speaking, I do think the Zoro v Mihawk fight will most likely be the B-plot of a few chapters while Luffy is busy fighting someone else like Blackbeard or Buggy or something. Oda likes to have multiple big fights going at the same time, so he can cut between all of them. Personally, I don't like this style, and think Oda works best when he just sits down and focuses on ONE fight at a time. But, Oda seems determined to try and make this work. And...this is going to upset people but...I could honestly see Zoro and Mihawk's final battle ending up being a long staring match capped off by a single blade stroke. A Iaijutsu/Single Stroke Battle type of fight. I know that would disappoint people...but I could see Oda doing it. But, even that could be slightly cool. These two warriors just starring down each other not moving in the middle of the chaos of a battlefield all around them. Just measuring each other's strength and waiting for each other to make the first move. There are ways something like that could work. So, if we end in a iaijutsu match, this final battle might not even take that many chapters. We might get a few B-plot build up chapters and then ONE focus chapter where Zoro and Mihawk finally strike at each other.
Blackbeard is probably the character that I'm personally expecting the most backstory and action out of. We need to learn about what caused Blackbeard to be like this. How he grew up. WHAT kind of creature he is. What his Devil Fruit actually is. What his giant circuitous scheme has been this entire time. And I imagine Oda wants to do one big fight between Luffy and Blackbeard. I don't know if that's the FINAL fight, but it potentially could be. I could be completely off base with this, but I think we're probably going to be getting the most amount of time with Blackbeard out of all of these characters on the list.
