I don't really think it's that big of a deal whether Togashi had planned or not planned the Chimera Ant story. He's got enough elements and plot threads layered throughout the series that he can build the Hunter world for years. The only problem is whether he will stick the landing. The way I view Hunter X Hunter is it's Togashi's manifesto of how to write a shonen series, he takes tropes that have become cookie-cutter standards of the genre and then pushes these elements into a more smarter and sometimes painfully real direction. Maybe I just revel in this form of shonen deconstruction, but a part of me likes the idea of Togashi looking at the mass "new world" hysteria that has consumed Jump and thinking "sure why not". My only criticism with the whole dark continent angle is that by opening the world up this dramatically the overarching stories that have been building up for the past billion years may not have that same punch to them depending on whether the Hunter world is actually existing on a microcosm of the real world.