After leaving Skypiea, the Roger Pirates tracked down the remaining three Road Poneglyphs.
By the time they reached Water 7, Roger had gotten a copy of Big Mom’s Road Poneglyph.
Zou’s Road Poneglyph was the last one Roger found.
It is revealed that during Roger’s journey, a Road Poneglyph was located at the Sea Forest next to the one with Joy Boy’s apology.
Luffy already owns 2 Road Poneglyphs, the ones from Big Mom & Zou, and is aiming for the Poneglyph of Kaido, leaving only the Poneglyph that was hidden in Fish-Man Island.
So the question is, where is it now??
There are two possibilities, either Neptune entrusted it to someone else or someone took it already.
I doubt someone took it already by force for three reasons: first, aside from Kaido & Big Mom, there is no other major Pirate who could have taken it by force, second because Neptune didn’t say anything about it to Robin like it’s missing or stolen or something and third, even if someone wanted to take it by force, they would just make a Copy, no need to take the whole Poneglyph.
So obviously whoever took it, it was for safe-guarding, not because Fish-Man Island isn’t a good place, but because after the start of the Golden Age of Piracy, Fish-Man Island is endangered by countless Pirates & whoever took it doesn’t want it to fall into the wrong hands and keep it as assurance that only the worthy one would find it.
There are only four characters who could have taken it:
1–Whitebeard Pirates that took Fish-Man Island as their territory may have taken the Poneglyph and hidden it.
If Whitebeard and his crew had taken the last Poneglyph, what would have happened to it? Well the first possibility is that they would have put it somewhere very secretive so that only they knew where to find it, and they currently still know where it’s located. This possibility ties in to how the remnants of the Whitebeard Pirates play a part in the Wano arc.
2-The second possibility is that Blackbeard has it.
As we know, following Whitebeard’s death and the events of the Payback War, Teach went on to conquer most of Whitebeard’s former territories, so the likelihood of Blackbeard and his crew stumbling upon it while taking territory is plausible.
Another thing to point out is that Blackbeard was on Whitebeard’s ship, which means he probably had some insider information on where it could be, and during the timeskip he could have snatched it. It could have even been the main reason for the Payback War.
3-Then there is Shanks which is an excellent choice.
It’s possible that he took the last Road Poneglyph and is protecting it, and is waiting to give it to Luffy when time is right, after all he seems like someone interested in Balance and supporting the New Generation/New Roger.
4-And finally it could be Scopper Gaban who took it to Lodestar Island which honestly makes sense.
It makes sense that just like Crocus and Rayleigh, Scopper is sitting somewhere where he can support the next Pirate King and Lodestar Island is an Island that must be visited.
So it makes sense, that they need to go there because the last Road Poneglyph is there.
*Theory by Salah Eddine