Interesting, since its supposed to air on the episode that was gonna be broadcasted March 27th that would be Episode 492 ( unless the Toriko/One Piece special was gonna take ONE PIECE's slot for that week) which makes sense since Episode 492 following the One-chapter-per-episode route count would be chapter 582
! Beginning of flashback .
Which leaves the anime with 15 manga chapters to adapt, because after those 15 chapters we got the
And knowing Toei they are gonna make more of those 15 chapters, at least they have to complete the filler adventures of each of the Straw-Hats, as far as I recall each was in a cliff-hanger, no?
Anyway, the least number of episodes any OP opening has had was 14 with Brand New World But maybe that was because of the We Are Straw Hat version opening? but I can't see Toei making a new opening for just other 15 episodes..
This leads me to believe that the opening will be for around 20-30 episodes:
1.- Toei will use the usual 1-per-chapter route with the next 7 chapter's adaptation to the anime. 7 episodes.
2.- The next couple of chapters are gonna be handled kinda differently, i.e. the strawhat's parts are gonna get expanded, ending the filler story from each character. Probably taking 1/2 episode per character instead of just using the manga's 1 or 2 pages for each character. So I would say around 10-15 episodes.
3.- Finally, I am fairly sure too that there should be some kind of flashback, hopefully not many episodes, but a quick compilation of how the Straw Hats get to join the crew is almost a certainty…
If I recall correctly the Enies Lobby filler was 1/2 episode for every character except for Luffy and Nami which got an entire episode for themselves, but I could definetly see Toei using an entire episode per Straw Hat flashback.
So.. for flashbacks around 1 to 9 episodes.
And adding up the numbers its a low of 18 to high of 31 episodes, so probably around 26-27 episodes, that's just my estimate
As for the opening itself my guess is it will focus on what the Strawhats have been doing, have some moments with all of them together, probably flashbacks of how they meet, I would be happy to see Toei using the Wonderful Encounter color ilustration, of course animated (Kinda like how they have used some illustrations in other openings) because its one of my favorite color spreads ever, it captures the essence of ONE PIECE so well
Assuming we won't get another entire season of filler, which is not likely at all, unless they create a out-of-timeline filler a la BLEACH out of nowhere the next season is gonna be
! the time-skip!! and I would be very surprised if We are isn't used as the opening song for this.