@Galaxy:
No, he isn't fully unpredictable like you're trying to say.
Yes, there are major story elements like those plot twists that nobody could have seen coming exactly, but when it comes to major decisions like new crewmates, the final villain, and the major stuff that will happen come end of series, yes, stuff is predictable because of the hints and patterns he's put down throughout the manga.
Random unpredictability leads to bad writing, like the end of Naruto.
Saying stuff like, "Luffy will fight, Teach, Kiado and Big Mom at some point", isn't making a story predictable though, it's called story mapping. There's a million and a half ways Oda can have Luffy meet those characters and knowing this man, I have a feeling it will be through a means none of us see coming. A more recent example would be Doffy saying "slaves are out and smiles are in", back in Sabaody. At the time none of us knew what smiles were, but they were a plot element Oda ended up using for a future arc, hence the true purpose of what Doffy said was unclear for nearly 5 years.