Bumping this thread because I feel this still an interesting conversation lol. But to restate what the OP is saying, when you take a lot of narrative implications between Blackbeard and Akainu, they are inherently tied together. This is the same mindset I had for several years when guessing a Vegapunk arc was coming. We needed something because Kuma, Kizaru, Sentoumary, Bonney and Vegapunk all had core elements that needed resolution.
Same thing Is tied to Blackbeard and Akainu if Oda is meant to tie up lingering plot threads with them:
- Again, they are the main orchestrators of Marineford and the deaths of 2 major characters respectively, Ace and Whitebeard.
- Chapter 650 focuses on both of them specifically as a cautionary tale from Jinbe on what to expect going into the New World
- Kizaru and Aokiji are main focal points in the current plotline. Where Garp now has lost and Kizaru(and even Saturn) eventually losing, promping Akainu to take action against both Blackbeard and Luffy, including Sword for the former.
- Aokiji himself has narrative history with Akainu, and that may be explored, which would require the 2 to be in the same arc somehow
- Cross Guild is tied to both factions via Mihawk's Marine Hunting and the underworld business Buggy is using for hunting Marines, which Blackbeard is taking advantage of. I.e. the Underworld is a connective tissue between both Blackbeard and Akainu in a way
- Shanks himself has narrative tissue to the Marines (Ryokugyu states they intend to fight the RHP), Blackbeard (their history), and Luffy (obviously), prompting him to also be involved at some point but when would be anyone's guess
- Dragon's obvious past relationships with Garp and potentially Akainu may prompt him being involved at some point too. Not necessarily the full RA, but him and Sabo make sense especially for Sabo to get a cathartic resolution about Ace if both Akainu and Blackbeard are defeated.
- Finally, as mentioned, it comes down to battle logistics. What escalation comes after an Admiral? The Fleet Admiral no? A Yonko above Kaido? Conversely, how do you escalate for Zoro between Shiryu and Fujitora, or Sanji between Laffite and Ryokugyuu if these are their battles? A free for all then? 2 sequential battles between 1 or 2 arcs?
Smaller topics, but you can even take certain character relationships further:
- The Big Mom Pirates are tied to Elbaf inherently by virtue of Big Mom herself. On top of this, Pudding is kidnapped, so you could make the implication that this plotline with Blackbeard may come to head at Elbaf with Katakuri, Smoothie, etc.
- Neo MADS is obviously tied to Vegapunk, who will head to Elbaf, and they share history with the BMP, who are now tied to Blackbeard.
- SWORD is very clearly the narrative thread between the Marines and Blackbeard, so Koby, Drake, etc may end up being involved soon. Interestingly enough, Smoker was not present during those events but he IS friends with Kuzan, this should be present whenever his story is tackled.
And finally, to me, this is the big "Incident" the SHF ends up being a part of, the complete defeat of the current Marine establishment and a Yonko
By this point you are left with very few threads to pull on and One Piece resolves some of its biggest endgame roadblocks in one fell swoop....