Marco showing up has pretty much convinced me at this point that the final Road Poneglyph is on Sphinx Island.
The “missing” Road Poneglyph was from Fish-Man Island in Roger’s time. 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.
After Roger’s execution, Whitebeard took over Fish-Man Island until his death, where Big Mom instead took it over.
Big Mom had a Road Poneglyph in Roger’s time, and didn’t have two when Luffy was in Totto Land, so she obviously didn’t take it, meaning that the final Road Poneglyph disappeared sometime during Whitebeard’s rule.
Sphinx Island is a non-World Government affiliated island under Whitebeard’s protection, with a special, hidden village, that Marco has been protecting ever since Whitebeard’s death.
It’s likely that Whitebeard secretly moved the Road Poneglyph to Sphinx in order to protect Fish-Man Island from Poneglyph hunters, knowing that it’s an island that’s so under the radar, and so safe under his protection, that nobody would find it hidden there. This is the meta reason for Marco’s arrival: he’s going to take Luffy to the final Road Poneglyph after the war.
Marco is going to play a role in the plot outside of just fighting and leveling the battlefield between the forces. I think outside of the Roger Pirates, Marco must have extensive knowledge about the world and One Piece. Considering his relationships and who he knows there’s no way that Marco is just a fighter moving forward.
*Theory by mcallisterco