Uh, you do realize that One Piece put into novel form would be about 3-4 books, at the most, right? The only reason it has taken so long is because it's a comic book. Oda has to draw everything rather than write about it. In all honesty, the story hasn't been that long. Oda could have easily planned everything that's going to happen already.
There's simply no way he planned everything. He expected the story to last 5 years. It's now DOUBLE that and shows no signs of slowing. Does he plan big-picture ideas? Yeah, I bet he does/did. I think it's fairly obvious given the attention given to Laboon and the Rumbar (whatever you want the spelling to be) Pirates in Blue going so far as drawing a flag for them, that he intended to bring them back in SOME way. Does that mean he thought of Brooke? DOUBT it.
But since he loves Nightmare Before Christmas and names/bases characters off of many creatures from that universe, I'm sure he envisioned involving a skeleton with the crew at some point in some form.
But then you've got the early Baroque Works shit which is just a fucking mess.
Somehow…..the mastermind of a criminal network....that stretched across GL........that involved some of the most deadly assassins known to mankind........also managed to include in its ranks the princess........who mysteriously went missing........of the very same kingdom he was attempting to put into his hands........whose disguise was a scrunchie.........
Yeah.........no..........
I don't care how the story covered things up with the 'it's so secret that nobody knows anybody' etc.
Yeah, no.
That's actually one of the biggest and most painful plot-holes in the series. Not to mention the existence of Igaram and his god-awful hairstyle.
Oda CLEARLY hadn't thought that out.
But that's okay.
That was still early on and it's crystal clear that he does his planning WELL in advance now.
He has many of the basic ideas. Some he throws around, jumbles, adds, warps, etc. But it's clear not EVERYTHING in the story has been meticulously planned from the start.
BTW, I predict this arc will end with everyone throwing a party on Sunny to the tune of Binks' Sake.