How many Fantasy (or probably more like Pulp books that cover fantasy/horror/sci-fi elements) are must read books from the 1900's to 1923?
There are some that some would suggest (HG Wells, Jules Verne, Tarzan and other Burroughs novels, I would probably say the Fairy Books from Lang) but there aren't all that many out there that people recall or say are must reads.
I'd say that those that will be must reads 100 years from now will probably be just as minimal. I'd probably put money on Sanderson's books. The Expanse may survive with the media that came out for it (though, there are very popular series that came out in the early to mid 20th century that aren't considered must reads or must sees today, for example Blondie I understand has a movie series that has over 40 movies in it, but who knows of Blondie today except for the old people like me?), and...maybe Game of Thrones (does that one count though, as the first was released in the mid 90s).
Most of what we think are big impacts of today probably will be forgotten in 100 years.
(Ah, one other that I forgot from the 1910s and 1920s...perhaps HP Lovecraft's works also count as influential works 100 years later, though I think they would be more towards Horror than Fantasy/Sci-Fi, the influence on Fantasy Sci-Fi is undeniable). In that light, perhaps something OTHER than Fantasy/Sci-Fi may actually have a MASSIVE impact on Fantasy/Sci-Fi in the future.