It reminds me of what I hoped Lost would be... I trust Oda to stick the landing, which Lost writers definitely did not. But the same thing might happen and it would be such a disaster.
Lost was a few seasons long, with different writers, a TV show so depending on actors and their availability... The final crash was predictable and almost acceptable.
One Piece is one man's story... He MUST stick the landing, and I think he knows it, and I think he will. But I'm sure there is a lot of improvisation in it, and that's normal. Can you imagine planning all of it at the beginning, and then writing and drawing everything like a robot for 30 years ? He would have shot himself year 10 lol...
I see Oda like I see another comic book writer I admire a lot : Grant Morrison. He has written long runs, with the same methodology as Oda : he knows the beginning, the themes of his story, the plot, the twists, and the ending. But he gets there with a lot of improvisation, and especially by "listening" to his characters. He gets to know them as he writes them, and this may change some elements in the middle of the story. He might create new characters to play the role he had in mind for others. But he always ties it all together, themes and characters, in the end. And that's what I think Oda is doing and will do in the end...
What's mind-blowing about Oda, is that he's doing it on a very very very long run. Take any Marvel or DC series for 30 years, and it's retcon upon retcon upon retcon, contradictions everywhere. You go with the flow because that's the nature of the beast but when you see One Piece... Very few retcons, and all them very elegant. To the point where we can't even tell if they're really retcons lol.
So, were Imu and Rocks planned from the beginning ? I don't know and I don't care : )