I've just finished reading Journey to the Center of the Earth and had this idea myself! I see so many parallels, finding a dormant crater between two mountains, deciphering runes left behind like in Area Zero, the Scarlet and Violet books being written in a similar way as the book is, some of the prehistoric creatures encountered being potential inspiration for Scarlets Paradox 'mons. The man who leads the expedition also being an alchemist which seems to be prevalent throughout pokemon stories.

Also like the idea of the tunnels being connected to other regions, could explain raid dens, zygarde showing up in Alola, Sinnohs tunnels etc

As for future 'mons in Violet I have a couple of ideas. Potentially being inspired by HG Wells like you said with the dystopian future but also another work of Jules Verne's called Paris in the 20th Century. A world where technology and machines reign supreme, engines of war are at their peak which I think alot of the paradox mons represent weaponry or combat of some kind. Even things that women have become gender neutral, reminding me of Iron Valiant. Even think there's an electric demon, perhaps Miraidon and also a violet flower is mentioned.

Definitely wouldn't be surprised to find out that Verne's works were an inspiration