Well we can start with the safest guess: Kizaru
1. Imo, Kizaru goes down in Egghead and so do major members of his fleet. Saturn survives the arc and the crew gets away with some of the Punks, CP0, Sentoumaru and Bonney. Either Blackbeard's group or Saturn take the Serephim
2. This leads us to 1 of 3 places: Fullalead (to deal with the whole Koby/Garp scenario unless that carries over somewhere else), some Underworld related arc to find this Burned Scar Man, or more simply Elbaf, because thats where Kid's destination was. Really this is the "big unknown", we dont know where we go after Egghead.
3. Elbaf imo is more self contained and adventure oriented, focusing on resolving the hypothetical Warland conflict, learning about some of the bigger world mysteries (i.e. the Ohara books w/ Vegapunk) and allying with the Giants.
4. The fight for the 4th RP occurs after this. Imo, this is either at or right before Lodestar, leading to the defeat of the burned man (if they are a villain) or Blackbeard (if they are not). Really I think its important at this point for CG to be out of the running as well. Not Mihawk mind you, but Buggy's major role.
5. At this point imo is where Shanks plays his biggest role in the story, unveiling his true goals at or near LT. Imo, I think
@ArturCantSpeakJapanese made a great point in another thread that in Japanese, Shanks' wording on obtaining one piece is apparently far more aggressive than Buggy, BM and Kaido's wording about it. This can be seen here:
6. Whether or not Shanks steals One Piece (maybe its his purpose or ulterior motive, but needs Luffy for "something"), this leads us to the Final War. First is the war with the Marines (Akainu etc). Then comes the war with the WG, God Knights, Gorosei, etc. 2 halves of a larger whole.
Tldr:
- Egghead
(Kizaru)
- Fullalead/Underworld or just Elbaf (
New Villain/Elbaf/Rocks maybe)
- Burned Man arc and/or Lodestar (could be the same) (
Burned Man/Blackbeard)
- Laugh Tale
(Blackbeard/Shanks)
- Final War
(Shanks/Im/Akainu)
Imo, at least 5 major sub arcs (including Egghead) because Kid and Law had destinations before Lodestar.