In theory, we could still get a few minor arcs. Depending on how Oda wants to do it, Lodestar would be a good candidate for a quick minor arc. Although, that could end up getting the full arc treatment. Dealing with the Man Marked By Flames and getting the final Road Poneglyph could work similarly. Or if we want to see the Revolutionary's side of what happened during the attack on Mariejois for a few chapters. Or maybe a little bit of what Blackbeard's been up to. We could get a "Reverie" style minor arc, once we finish Elbaph, just to see what's been going on in the rest of the World. But, there's also a pretty even chance that all that stuff will stay in the background of the story, unfortunately. And we still need to find out what happened to Law, and Bartolomeo, and maybe even Kid. There's a lot of things Oda COULD do, but I'm just not sure how many of them he'll actually get around to.
As far as how many major arcs are left? Well...I think we can all agree that there's at least going to be ONE more arc, right? At the absolute minimum, we'll have one more arc.
And personally, I think it's pretty possible that the next arc WILL be our true final arc. One last arc...but composed of several different "stages" and locations. Lodestar to start off, leading us directly into Laugh Tale, which will lead us directly into...some other surprise twist location. Possibly the true Underworld, and/or the Moon (which are probably more connected than most people are expecting). And then a little epilogue after the fact. But, I'm not expecting much epilogue, frankly. I wouldn't MIND a whole volume of epilogue, say about 11 chapters worth, and I think that'd be the best case scenario. But, realistically...any epilogue will probably be pretty short. Maybe one extended chapter's worth. That's just how Shonen Jump endings work.
I think Oda is gearing up for the ending. Which I think is coming sooner than most fans think. He wrote down the answers and released them into the ocean with the real-life One Piece. That says to me that he's ready to say goodbye to this story. It'll probably take longer than he thinks to end things, but I think the end is coming sooner rather than later.
There's also the problem of escalation. We're already fighting Imu, the secret ruler of the whole World, right now. There's...not a lot of other threats that Oda can really give us to top that. Blackbeard's the most obvious one who COULD grow into a threat that's bigger than Imu. Shanks is...plausible, but unlikely, in my mind. The Man Marked By Flames doesn't really seem like a physical threat, since he's mostly "running away", so if he WAS a "villain" for a arc, he'd probably have be a secondary villain for a bigger threat. Like maybe, the Strawhats have to deal with Blackbeard AND the MMBF at the same time. I do think we're going to end up getting a surprise final villain, whether it's a Demon King or some sort of "Dragon King". But, that'd be saved for the very VERY end. So, if Blackbeard isn't the final villain, that means we have two more arcs, max, due to just running out of "worthy" antagonists.
The only real x-factor in my mind is Hachinosu. Oda seems to have Hachinosu on his mind, recently. Brook's song in the latest chapter "Ya Better Buzz Off"/"Mindja Own Beeswax" seems to be referencing the "Beehive Island" and it's history as a penal colony. And, in theory, Hachinosu might be where the last of the 9 Rio Poneglyphs are located?
There probably is a important reason why Blackbeard and Rocks were both so interested in this island. So, alongside a showdown with Blackbeard and saving Garp, there are a few reasons to go to Hachinosu. I'm just not sure if we actually will go to Hachinosu or not? Because if the Strawhats DO go to Hachinosu...that WILL be it's own separate arc. Fighting ALL the villainous pirates on Hachinosu would be a big enough deal to warrant it's own larger arc. Which adds a lot more time to this manga and I'm not sure if Oda's interested in doing that.
So...maybe instead of going to Hachinosu directly, they just end up saving Pudding or running across Perona and Moria? That way they can still get the information they need off the Rio Poneglyph, but they don't spend a lot of time on another arc? Hachinosu I could see going either way. Either we just learn more about it in the background of the story. Or it's going to get it's own focused arc.
But, for right now...I'm leaning towards there only being one more major arc, myself. Blackbeard will probably effectively be our final villain. The lurking threat in the shadows that no one took seriously enough that grows into a cosmic menace, or unleashes one that he can't control. But, who knows? I honestly didn't expect Elbaph to go on this long. Or for THIS to be where we seem to be dealing with Imu. So, chances are, Oda's still got a few curve-balls left for us, yet.
