There is no way the WG goes down next arc lol. Half the reason Laugh Tale, One Piece and the Void Century even exists is to prop up their relevancy to the endgame plot. Imu is literally the last known or designated villain in this series without a design reveal.
Id say in the least, we have 3-4 major arcs left:
- Elbaf
- Lodestar/Laugh Tale (we may need to separate them)
- Final Battle against the WG (wherever it takes place)
And really, the only 2 big scenarios I can see from this is the following:
- Oda goes the "simple" route where until the end, only 1 antagonist loses per island. I.e. (just an example; you can flip the following based on your personal bias) Akainu and his Marines at Elbaf, Blackbeard at Lodestar, Shanks at Laugh Tale, Imu at Mariejoa or wherever
- Oda picks up the pace, where Elbaf is actually a grand war between Blackbeard, Shanks, Akainu, Luffy and CG, and that simply pools into Lodestar/LT with whomever survives this part. It is important that Blackbeard or Shanks get "close" to LT in a sense because Luffy needs to surpass them to become PK. Then after this, still another fight with the WG/Imu.
So yeah, one can take signficantly longer than the other, but still both options result in years of material before it ends.