I'm in the middle of the "Wheel of Time" books. That must have been the most planned out series ever. Or reworked like a million times. At least, that's what I gather from people who have reread the series and praise the amazing foreshadowing in the first half of the series (even books 1 and 2).
For those who don't know, "The Wheel of Time" is 14 books plus a prequel.
I think the writer had a vague plan at the start, but nothing beyond that. This is pretty much proven out in books 5-10 where he rambled on and on about basically nothing. When called on it, he said he'd do better and he did on the next couple, but then fell ill.
He managed to write up his ending notes and hand his notes to Brandon Sanderson before he died. But I don't think those notes were written before that.
When I wrote my book, I intentionally left a few bricks lying around in it, so that I could make them look like foreshadowing later on. Really, I have no idea what they will connect to, just that they're not connected to anything right now.
Several of the loose ends are already planned to connect to books 2, 3, and the (presumptively) conclusive book 4, but there are others that I just threw in as an investment that might pay off later. But no reader would know for sure whether any given loose end is a forward link or just a brick, until the series is pinched off and it's still left hanging.
For those who don't know, "The Wheel of Time" is 14 books plus a prequel.