Ok, this theory contains spoilers in Manga Chapter 817, turn back now if you have not read it or Anime Only.
So in the latest chapter we ran into a new poneglyph different from all the others, being a deep red color (I would think crimson color). In the chapter it says it is for a different purpose (
http://readms.com/r/one_piece/817/3248/16this link is to page 16, page 17 says it is for different purpose) and I think the purpose is to lead to raftel, and this red poneglyph is a rio poneglyph (one of many).
Breaking it down, Crimson wikpedia definition "Crimson is a strong, red color, inclining to purple. It originally meant the color of the kermes dye produced from a scale insect, Kermes vermilio, but the name is now sometimes also used as a generic term for slightly bluish-red colors that are between red and rose.", also here are some cultural references Cultural references "[edit] The King's Royal Hussars still wear crimson trousers as successors to the 11th Hussars (the "Cherrypickers"). In Polish, karmazyn (crimson) is a synonym for a Magnate, i.e., a member of the rich, high nobility. In texts of the Bahá'í Faith, crimson stands for tests and sacrifice, among other things.[35]." Deep Red or crimson, I believe also can have the meaning of blood being similar in color.
So if the red poneglyph is about blood, and Rio is Portuguese/Spanish for River (like the flow of the river), we have Blood Flow.
So the rio poneglyph that Robin is searching for is not a single master poneglyph, it is a series of special poneglyphs. You have to follow the flow of the red ponepglyphs to find out the true story of the void century and following each rio poneglyph to the next will lead you to the truth and to raftel.
I suspect each poneglyph has a marker leading you to the next point, so the next rio poneglyph will be in Wano, and will lead to the next rio poneglyph. I do not think there are like a bunch of these rio poneglyphs just a short few in difficult to reach locations, like with Zou an island never in the same location always moving, and then Wano a country able to defend itself from the WG keeping its original culture. So, there must be one more leading to raftel and to the truth of the void century.
Let me know if I am crazy or maybe onto something. Cevirebiliçek olan varsa lutfen