You say that like its such a small thing left lol. It took 350 chapters to finally tie in and start dealing with Big Mom and Kaido.
The World Government is not "small". You have multiple sub-factions within its grouping:
- The Leaders: Im, Gorosei, Kong, whomever else runs the show in the World Government that we have yet to meet.
- CP-0: The agents that protect the Tennyruubito
- The Marines: Akainu, his Admirals and military (additional VA's we haven't seen, etc)
- The SSG: Vegapunk and his science team, replacing the Shichibukai.
Then factor in the pure fact that we know next to nothing about Shanks and Blackbeard. We have no idea if Shanks is "all good". He was shown talking to the Gorosei and people keep denying that he isn't being weird or shady. These 2 are vastly more important Yonko that BM and Kaido. Do I expect another 400 chapters to deal with them (if Shanks is bad)? Definitely not, but their stories will have to be built up, just like Big Mom and Kaido. They will (probably) factor into this huge war with the WG somehow.
Then tie in virtually every other additional group that has the potential (I won't say guaranteed) to participate in this war:
- The Revolutionaries (Dragon, Sabo, The 5 RA Commanders)
- Former Villains (Crocodile, Enel, Moria, possibly Doflamingo, possibly Magellan)
- Former Shichibukai (Mihawk, Hancock, Buggy, Kuma, Weevil)
- The Supernova (Kid, Killer, Law, Drake, Hawkins, Apoo, Bege, Urouge, and Bonney)
- The SHF
- All allying and enemy Kingdoms taking part in this war
- The Underworld
- The Level 6 Inmates that escaped during the Paramount War (Still unaddressed)
- Former Whitebeard/Roger/Rocks pirates or any other Pirate group willing to take part as allies or enemies.
Like dude, its not small. Not at all. Oda has entirely end loaded his story. Either he just skips or drops tons of things or he will spend years drawing it out. Considering how much he keeps hyping it up, I'd prefer to think it will be the latter.
Edit:
IMO, after Kaido/BM:
- Blackbeard (Laugh Tale)
- Akainu (New Marineford/Mariejois)
- Im (Mariejois)
- Shanks (God Valley)
Is the order/rundown the antagonists Luffy will face. Blackbeard is mainly a roadblock to Luffy becoming PK, not warring with the WG. Finding OP and learning of the Void Century/True History is pretty central to even challening the WG (whom Akainu/Im will comprise of Luffy main villains there). Mihawk needs to be fought last for Zoro and Shanks is virtually the only group that can face people while also giving Zoro the battle he deserves because Mihawk and Shanks are associated with each other. Shanks is, pretty much confirmed due to his conversation with the Gorosei, not good like people think. There is a reason Oda drew that double spread as a shock. It's supposed to be shocking, because WTF is going on with him. He's also the same age as the God Valley incident. I also think God Valley is the last location this series will actually visit. It was hidden for a reason after Rocks failed to do what he wanted to do. Why?
I do NOT think that is coincidence.