Well, in the sense of experience, we certainly can't attest to that we know until after the Yonko arc (possibly).
To me, One Piece is just 1 big story, so I believe a lot of answers have been subtly revealed, and in some cases explicitly delivered to the readers. 800+ Chapters is a lot of Chapters to go by and still not have an idea, so, for most questions, I believe it's a matter of searching.
Regarding what we know concerning Raftel, the hints are pretty abundant in my eyes.
The entire plot focuses on Freedom (Luffy's motivation to be a Pirate); the lack of Freedom results in Slavery, Inequality and Prejudice.
In this quest for Freedom, he's come across these obstacles, and when he sees something he won't put up with being enforced on others, he can't help but stand up to it.
Luffy as a character is somewhat representative of the will of the people (their innate desire to fight back) against these confining practises, more so, he's like a de-facto advocate for the weak being that he's strong. The weak will capitulate, Luffy who is strong will fight back, and this strength of his even inspires the weak to fight; he's like a revolutionary as we see in:
- Dressrosa
- Sky Island
- Fishman Island
Besides the strong theme of Freedom, there's also another major aspect of the story that holds just as much importance, and that's the Poneglyph which is very central to the story. It's easy to get swept in by the fights and comedy, but we need to remember what the story is centred around, and that's ultimately the history of the world.
The WG wear a facade portraying themselves to be freedom fighters, upholders of justice and protectors maintaining order which they try to represent with their neat, clean and white ensemble. However, we know that is mere aesthetics, their foundation is built on lies and the organisation itself is terribly corrupt.
We even see the lengths the WG go to into suppressing the truth (decimation of the Ancient Kingdom and Ohara), we can be sure everything transpiring will culminate into resurfacing of this truth, and that will surely shake whatever foundations this WG is built on, and eventually result in a revolution.
Going back to what WB said, he said once it's found:
- Someone will carry the history of the world on their shoulders.
- and wage war on their shoulders.
For me, everything points to the history which has to be at Raftel based on the importance and significance it is given, and more so since Roger said that's where he had to deliver all the messages from the Ponegylph.
The entirety of the WG's actions have been to keep their deeds during the Void Century secret and even tried to suppress the desire to go after One Piece (which we know is at Raftel). The only thing the WG could be protecting by discouraging both these acts surely suggest of one thing, and that's the fact they reveal what they don't want known to the public which is the history - the suppression of this history is key to continuum of the power and control they have.
There's probably more to Raftel than just knowledge, but I'd assume that it's the critical significance of Raftel.