@Mrs.:
Well, we've had in the sky and under water, so those are out. It's not on the moon 'cause Enel was there and it has to be on/around GL. Maybe it's a secret passageway to an underground paradise.
IMO we're going too far now.
It doesn't have to be a special island in a way like being in a strange place or something.
It could just be a normal island, but a cool-looking one.
Besides, i think that once they get to Raftel, they've already had all the tests they could get. Raftel just holds the One Piece, probably nothing more to it.
putting simple tasks there would be pointless, and I cant think of anything else to do there (outside of maybe finding the way to the treasure).
But Raftel could be a nice place for the final Luffy vs. Blackbeard fight.
I'm pretty much done on the subject of Raftel.
I don;t wanna think of anything else it could be.
I'm just gonna hold on to the things I have in mind right now, and I'll see how many of them are true once we get there.