Wealth, Fame, Power. Gol D. Roger had it all.
So what can he possibly leave behind that is so damn valuable that it can actually grant whoever finds it all of that the title of the Pirate King? His log book.
Now hear me out, Roger’s log book must have a lot written down. World’s secrets (ex: knowledge of the Void Century, Ancient Weapons, etc), treasure locations (ex: the City of Gold in Skypiea), along with all of his experiences and adventures. It’s not hard to see how a competent pirate with a hearty crew can easily conquer the seas with access to such information.
Just 2 random examples:
“Oh after finding Roger’s logbook on Raftel we learnt that there was a multi-billion berry treasure hidden near Pluton, yeah we used it to build 4 more and now we gonna blow up the Red Line because we can”
“How did we do all this without being noticed? Well turns out there are islands in the sky so we just flew to one and did this all over the radar. How did we get there? Roger’s log book.”
That random example is just to show what type of things can fall within the realm of possibility if the One Piece is Roger’s log book. Just use your imagination and you can see how it could be game changing.
My favorite part about this theory is the Log Book would truly grant someone everything Roger left behind together in One Piece and it would simultaneously only be One Piece of the puzzle since there is so much that can be done after attaining it.
*Theory by AstralDoctorLN