The whole “open Wanos Borders” plotline slash mystery was one of the absolutely most maddening things about the arc for me.
Like so Wano is introduced as an isolationist samurai country. When we actually meet wanoese people we see them to be old fashioned, to the point of being willing to burn things they don’t understand at the stake. Apparently there are laws against leaving the island, and a great many of the heroic Wanoese characters have “Opening Wanos Borders” as their explicit goal, so you’d think this could mean something like making Wano a part of a larger world, maybe fostering progress or something. But this doesn’t really seem to be of any real concern to anyone, and is undercut by Wano not feeling that consistent about it’s isolationism; like it’s not really strictly enforced or anything. Oden/Izo leaves with no consequence, as did a whole clan of people who went all the way to east blue in order to drag a piece of red string to connect to Zoro. No one in Wano seems to really care about their isolationism one way or the other.
I guess you can say Wano is isolated in the sense that, well, it’s hard to access geographically but even this is inconsistent. Oden might not’ve been able to traverse the waterfall but Kuinas clan and Orochis grandma sure did, and even post Kaidou occupation we have Luffy, Law, Ace, Green Bull and an ENTIRE MINK army slip into the nation; the only time Wano came close to the idea of an “impregnable fortress” was when the second in command of the beast pirates manually had to fly down and kick a boat off the waterfall.
Of course Wano is also NOW isolated with “closed borders” because it’s occupied by the beast pirates, but this is a wholly separate issue from the “closed borders” Oden wanted to open. The story doesn’t highlight the occupation as an explicit result of its isolationism – the occupation happens due to a combination of supreme carelessness and underhanded manipulation, it’s not going “this would’ve been avoidable had wano been more willing to trade/had made more allies/whatever”. Rather, Oden staying put as Shogun would’ve avoided all the nations misfortune. Still, the story equates liberating Wano with opening its borders, never mind its previous vague policies on the matter, and this at least ties in with the idea of going outside of Wano to forge an international alliance to liberate an isolated and friendless (well, save for the minks) nation).
Only in the end, Momonosuke declares the need for Wano to be able to be self reliant in terms of defense, and the whole international alliance amounts to basically a means to an end. The whole issue about “opening Wanos borders”, well, the characters didn’t really know what it meant, no one but Oden did, people just knew it was important in some way.
And so Wano doesn’t really change post Kaidou. Because in the end, after so many declarations that the heroic characters would open the borders, after a villainous character literally goes “neener neener neener, the borders are staying clooosed!” in the ramping-up-to-the-final-punch flashback, the protagonists choose to…not open the borders!! How supremely unsatisfying! But you see, there’s a good reason for that because “opening the borders” actually turns out to mean “unleashing Pluton upon the world”, dun dun dunnn big reveal. Which…has nothing to do with anything? Seriously, I cannot link this idea to anything in a narrative or thematic sense. I cannot supplant the spirited declarations of “We must open Wanos borders!” with “we must unleash Pluton upon the world!” and extract any meaning from this. It’s just another thing that means absolutely nothing to 99.9% of the characters, with an additional 0.09% knowing of Pluton, but being still no wiser as to Odens motives.
It serves as an “answer” to the “mystery” of what Opening the Borders, an answer that just fuels further questions, and not much more.
Ultimately, I have no idea what Oda was saying about Wanos isolationism.
TLDR: the handling of ”Opening Wanos borders” is like if characters in Skypea didn’t know why they wanted to ring the Bell, and when the characters find the bell theres a note saying “NB: Ringing the bell summons Uranus” and then Gan Fall declares that they shouldn’t ring the bell, and the arc concludes with the Skypeans/Shandians not really being on good terms and Cricket going “well, I’m sure the bell will ring at some point”