Here is my Analysis for how One Piece will wrap-up:
Well the way i see it, SHs have 2 Road Poneglyphs & gonna get the third one Now
So logically once they locate the Fourth One, all others have to do is also go there since with Luffy going there, all Four will be up for the taking
So we have the Idea that can trigger Final War (Road Poneglyphs / One Piece)
Now we need a Location, there is only one where all Major Players could clash & it's obviously Marine HQ/Red Line/Mariejois, it's the perfect place for Final War & all Manga was building up to this. If that's the case then this means Fourth Road Poneglyph is there
What could have taken Road Poneglyph from Fishmen Island to Mariejois? Let's check what we know so far:
1. Only ones who were shown to find Road Poneglyph on Fishmen Island are Roger Pirates
2. Fishmen don't seem worried about it's disappearance suggesting someone they trust took it
3. It's most probably in Mariejois & made WG have good Relation with whoever took it
There is only One Character who fits all this, it's none other than Red-Hair Shanks, he is the one who knew about it, could have been allowed to take it by Fishmen & then he gave it to WG explaining why they are on Good Terms with him
Now why would Shanks do that? First of all, it allows him to stay in touch with WG since from their perspective they guaranteed no one can find One Piece, but that's exactly what Shanks wants, he wants that the next Pirate King cannot find One Piece unless he defeats WG forcing him to amass an Army. This way Shanks makes sure no one Unworthy Finds One Piece, no one who is not willing to fight WG will find it & no one finds it and be unable to use it similar to Roger due to lack of Power
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So how are Straw Hats gonna learn about Location of Fourth Road Poneglyph? It's also simple, we already said that Shanks is the one who hid it & he is the one who inspired Luffy to start his Journey, what did he ask from him to do? Reach his Level
So it makes sense that when Luffy proves himself, Shanks is gonna face him & reveal where Final Poneglyph is at.
Defeating Kaido is more than enough to prove Luffy & his Crew's Worth & he finally deserves to meet Shanks again.
However Shanks won't fight him just yet, he will challenge him to prepare for Final War at Marjorie & see who claims Pirate King Title
Such a War will be known to whole World ofc, with the prize being One Piece itself.
All Marines are already there, WG & Cipher-Pol are there, Revos will obviously attack, Hancock is the Damsel in Distress this time
Crocodile will definitely join this War as a Wild Card again, BB too & as i already said Red-Hair Pirates & SHs
(Depending on how Wano ends and it doesn't matter whether Alliance lose or not, Kaido or/and BM will join this War too
(I lean more towards BM since i don't see the need for defeating her this Arc & her Children don't like this Alliance with Kaido anyway, so they will just abandon Kaido at the end & leave)
Bonney & Kuma are already in Mariejoie, X-Drake is a Marine, Law will either continue with Luffy or fight for himself, Kid definitely wants to participate & rest of Supernova .... etc
Weevil wants to fight BB & Luffy so he will be there, Mihawk will be interested too i guess since he is Bored & everyone that ever interested him is there (Shanks, Zoro, Luffy)
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SHs always arrive late during Big Wars, this Final one is no Exception
What can delay them? There is one Island they still need to go to & that's Elbaf
I doubt there will be any Villain there because if Yonko aren't endangering that Island, no one can
It will be short Arc with a single outcome, add Giants to their roster of Allies
The whole story have been building up to this SHs/Giants Alliance (Little Garden, Enies Lobby, Dressrosa, WCI ... etc)
How are they gonna bring all this Alliance when they arrive to Mariejois? What's the biggest way to arrive they haven't used yet?
It's none other than Pluton that Franky can build
Oda will also add Lodestar Island somehow (Maybe that's where Luffy & Shanks meet)
And then we have everything ready for Final War
Luffy becoming Pirate King surpassing everyone present there, Zoro defeating Mihawk there for everyone to witness, Supernova prove themselves, especially Kid, Franky builds Pluton & arrives with it, Usopp will command an Army of 8000 & lead the Giants as a Brave Warrior, they will destroy the Red Line, Yonko & WG system is over, corrupt Marines are defeated & the worthy ones remain, Brook reunites with Laboon exactly there, Sanji finds All blues at same Spot ... etc
It's literally the place where Final Arc could happen & everyone fulfill their Dreams & everyone appears
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So IMO right after Wano, Final Arc officially begins (Meeting with Shanks --- Recruiting Giants from Elbaf --- Fighting in Mariejois --- Fulfilling all Dreams) and Zou + WCI + Wano + this Final Arc = Final Saga which Oda called Yonko Saga back when Dressrosa Arc ended