@Count:
I realize that Totland as a term in and of itself does not sound like a spoiler. But we should be aware that Viz the story was thought to be primed go from Dressrosa, to Zou, and then to Wano Country. It wasn't 100% confirmed of course, but the chat at the beginning of Dressrosa where Law first brings up Zou and Kin'emon asks if they could go to Wano Country too sets up this mindset. Along with there being a built up mystery about what happened to the Swirly Hats. So Totland arouses a bit of awkward suspicion. Nothing obvious, but something suspicious about how the story is headed. Water Seven isn't exactly the same because it just happened to be the next random island the Straw Hats went to without any context, while Totland is a complete detour. If someone finishes Volume 81, which ends on Sanji being kidnapped, it's reasonable to assume that they can put the pieces together that the Totland title card corresponds to traveling to Big Mom's territory with how they haven't heard about it before.
Purple can work, although I think a bright color fits Totland better. But I would LOVE a rainbow-colored header like Volume 83. And the terms for arcs and sagas tend to be interchangeable when writing series. The first term refers to an over-branching, continuing storyline in an episodic series while sagas originate from being stories of heroism in either Norse or Icelandic diction. And this whole thing gets even more complicated with how Oda specifically remarked about the entire pre-timeskip era of One Piece being called the Sea of Survival: Super Rookies Saga while the rest if The Final Sea: The New World Saga. So we basically have arcs in sagas of, well, sagas lol. Not to mention that there are still debates over which specific chapters arcs end/begin, and even if Water 7 and Enies Lobby are actually one arc or not with how interconnected they are (and if they are one arc, it would be a few chapters longer than Dressrosa). Unfortunately, we're left to assumption without direct word from Oda or at least Shonen Jump.
Honestly, I think we're overthinking how much consideration Viz puts into these titles. Especially since there is now a mere three volume distance between eastern and western releases, so they don't have enough foresight to give a proper saga banner anymore. For all we know, we'll have an arc about going to the island where Nekomamushi is recruiting Marco after Totland instead of going straight to Wano Country (not that a Marco-focused arc wouldn't fit in a Four Emperors saga, or arc, or whatever).
A lot of good points here. The New World is a lot harder to break down than before, everything now is less episodic than it was. When before, antagonists weren't really introduced until the saga containing them, here everything is so much more interconnected. We can be pretty sure both Big Mom and Kaido will be big bads at different points, but Big Mom and her crew first appeared at the end of Fishman Island and established themselves as a threat. From there I think a lot of people figured she was being set up as the next boss. I remember seeing speculation that that the Emperor Law wanted to fight would be her, because of course. But then it was operation Kaido, and it seemed like he would be the first Emperor fought instead, then Big Mom interfered in that in the middle of Dressrosa. Everything we saw of Zou suggested Kaido minions were there only, but then team Big Mom shows up halfway through the flashback and makes it her story again. Oda has laid out his villains and a lot of his locations far in advance for this part of the story, and is shaking them up at will. It's so much harder to say this is where the Big Mom part of the story starts and ends, and this is where the Kaido one starts and ends.
The first half never did this, it would be like first encountering the Cipher Police at Alabasta, having them interfere here and there and knowing they would need to be taken down, but not finding the reason to until Water Seven. The closest thing the first half did have was establishing Fishman Island as the next destination after Water Seven, then throwing everyone's expectations by doing Thriller Bark and Marineford first. It's like a narrative version of the Unspoken Plan Guarantee: the more we know about the Strawhats' next destination, the less likely they are to arrive unmolested. (Consider also, how Oda established three possible destinations after Fishman Island, then brought Punk Hazard out of the left field.) We presumed we'd been given a roadmap of the next few destinations with the Kaido plan, but we were kinda naive to think it would actually be followed.
This does kinda make it a hard line to draw, whether you think a detour is an obvious enough narrative device that the cover can suggest it, or if you'd rather avoid any chance of spoilers at the cost of monolithic arc lengths. I doubt we're the only ones thinking hard about it, though. I doubt Viz would have started doing these if they weren't going to give them some thought. As I've said before in this thread, book design doesn't happen accidentally, and especially not for things that are going on the front cover, the first thing a new reader is going to see. I suppose the difference is that we're looking at them from the perspective of fans dedicated enough to sign up for a specifically One Piece based forum, while the people making them are a marketing department pushing for sales and people picking the volumes up off the shelves.
I'm beginning to think there's not a lot of official consensus about arc/saga designation, and what the best way to subdivide the series is. Pretty sure the Japanese terminology for "part of story" (編, hen) can be translated as either arc or saga (someone with more Japanese please correct me if I'm wrong). Oda himself as you mentioned cut the series in halves. WSJ is advertising the Four Emperors arc/saga. Viz sometimes divides by arc, sometimes by saga, but calls them all arcs in their material. Different Japanese ads have grouped volumes by arc and by saga, and we, the western part of the fanbase have our own ideas. And even together, as you said, we disagree about classification and where things exactly start and end (I mean, Thriller Bark feels like it ends halfway though chapter 490 - if you say the arc covers only up to 489, you miss out on the closing narration and the thing in the fog, which is a great conclusion, but if Thriller Bark goes right to the end of chapter 490, you have the reaching of the Redline and the finding of Camei counted as part of a storyline they have basically nothing to do with - but imagine arguing and explaining that point to anyone, or putting it on the wiki).
The only thing we all really seem to agree on is that One Piece kinda needs to be subdivided, just due to its sheer size. Compartmentalizing parts of the story makes it easier to remember and discuss and less intimidating for newcomers, as well as offering them good jumping in points, especially if they're switching over from the anime or something.
(A header for Totland that arouses suspicion that things won't go as planned, or that the map set out isn't being followed exactly could be done deliberately though, as a way of building atmosphere - Oda himself basically did this with the titling of volume 59/chapter 574, which confirm outright to the reader that Ace's wounds are mortal before the story shows it. I remember seeing one ridiculous person decry this as a spoiler, and cite it as a reason we shouldn't bother translating chapter titles, instead of accepting that we're intended to know the situation is hopeless going into it. The difference I suppose being that Oda personally was responsible for that one, while a premature Totland header would be 100% Viz marketing team work.)
Ahh, I've rambled. Sorry for the wall of text. Narratology and bookmaking really get me going.