Since it's almost definately sure to have been covered before but;
Has anyone had the idea this could be neptunes trident or crown? I know it's a long shot but I have a feeling Davy Jones will be modelled around King Neptune, and will somehow play a role in the fishman arc (aswell as being responsible for the massive aqua laguna, somehow the WG or Dragon have pissed him off).
Also not so much a prediction as much as it would be cool. I think that he would have OCD, that's he's been stealing ships to keep in glass bottles in his lair somewhere underwater to form some massive albeit pointless collection (possibly in a giant locker?). I honestly think he's also responsible for swallowing the Old Kingdom through some ridiculous bout of collectivism or protection, if anyone knows about Greek Mythology you know he was mean to protect the city of troy, which he kinda messed up.
If so then I reckon that he knows something about devil fruits and one piece. Davy Jones used to be known as 'Devil Jonah' the only proper mention of 'devil' in pirate folklore. If this is true though they probably won't go and find him again till the end of the story, and it could mean that 'D' stands for 'Davy' , it was in his will that they'd protect the sea's most valuable asset (One Piece) or something. This could explain why Devil fruit users sink to the bottom of the sea (if that's where Davy Jones, the Old Kingdom and One Piece are, but I can't think of a good way to explain that right now).
I he is this important, then not only would it give the whole TB more raison d'etre, it would also be a great time to introduce the main plot device (sneakily half way through the story)
I know Oda doesn't seem to be a great lover of mythology, so if this should be in another thread for shabby predictions feel free to move it.