Hello guys, my name is Jibon. i will tell to you my theory about where Last Poneglyph and what Raftel is.
12 years ago when i still in junior high school i played a game on my PSOne Console called "Beyond The Beyond" a Classic Japanese RPG where a 13 y.o boy named Finn play role as a hero to defeat underworld demon. shortly, in quarters of game the hero have to gather "4 ANCIENT TABLETS" 2 Tablets possesed by different kingdom, 1 hidden in the depth of forest an the last one possessed by "THE GOD" itself in His Temple, floating in the sky.
after we found all the Tablets, we have to put them in 4 different Temples (North Temple, East Temple, West Temple and South Temple), and then BOOM!! the cross like light formed from each Temple and an Ancient Prism like Temple show up from the bottom of sea which The Hero can travel around the world using it.
from my detail above we can assume that the two kingdoms similar to WCI (Big Mom) and WANOKUNI (Kaido), while Hidden temple in the depth of forest (ZOU), and The Last Road Poneglyph can be found in MARIEGEOIS possessed by Celestial Dragon / Gorosei / Im-sama.
and Raftel is located in the center of it.
from Raftel name i think it can be related to "Raft" or "Float", The Ancient Kingdom should be build on this Massive Raft like Island!!
sorry if this article hard to understand. because i still practicing english. feel glad if you want to read this theory