He initially planned to have Arlong park equivalent at chapter 3 but his editor told him to put it for later which I believe improved the story. Another story goes that he was planning to do a mecha manga after one piece and eventually decided to merge his ideas into the storyline, hence Franky. He now plans to do only One Piece.
Green also contains a lot of the initial character designs and naming patterns that changed with time
- Sanji was initially called Naruto
- Chopper was a zebra-raindeer doctor who walks on his hind legs and smoked cigars
- Robin and Franky's character would have been fused- Franky design whose only skill would be making weapons and hidden knowledge
- the shipwright would have been Merry's spirit
- Yassop would have been the Vice-captain
- One of Nami's early design would have been a swordwoman , another would ahve her axe crazy
- Oda had at some time planned to have Brooke or Zorro in Buggy's crew, Zorro initially would have only 2 swords
- Brooke had several design without an afro, one had him wearing a bandana with kamikaze written on it
- Kuma would have a steering wheel in his head just like Shiki
- Moria would have been a creepy priest carrying a bible with him
- Blackbeard have had several designs, including one with an eyepatch that appeared in an old colourspread(with Buggy and Shanks)
- Caterin Devon would have initially been good looking but Oda thought that Balckbeard pirates were the closest to being real pirates and thus should be ugly
- there is a recurring cat woman design that Oda still has not used,
- the Strawhat pirate final crew design seems to have been tampered with(so they say, I am not really familiar with those things) and something big seems to have been erased from the background. Some speculated that it was supposed to be Thousand Sunny, other thought its was new crewmates with some even proposing a giant mermaid even before the Fishman Island started, other thought that it included several big characters of Jimbei size, etc.
Even within the stories you can find several Chekov guns such as Rayleigh(some say that the anime initially wanted to remove Rayleigh's early scenebut kept it under Oda's insistence), haki, the cover stories, BB, the will of D, laboon, ryuma,dragon etc which indicate careful and long term planning.
The SBS, databooks and even colourbooks offer several fan bonus.(for more details about the Green Databook, check the corresponding thread in AP or listen to Greg's and Stephen podcast although that one lasted three hours) I think the first colourbook showed a colourspread of the red hair pirates showing characters that became more prominent when Shank stopped the war. Even some characters designs and names(the admiral, don flamingo, WB) are not just random and are based on real persons and arcs are reminiscent of other stories, all which indicate a degree of planning and really make the discussion more interesting. I doubt the editors or anyone would know about them beforehand