Off the top of my head when it comes to the genres I read the most:
-The Girl with the Dragon Tattoo. The nordic thriller genre was highly prolific well before it, at least since the 1960s, but Larsson popularised it to a wider non-Nordic public by introducing cartoonish, exaggerated, violent, movie-like elements that contrasted with the tradition both of Nordic thrillers and of crime novels in general, and formed the basis for many successful writers that came afterwards.
-Battle Royale. Pulp, extreme and theatrical, it is a landmark of dystopian fiction that IMHO deserves to be in the same realm as 1984 and Lord of the Flies (not necessarily in prose quality, but part of it is because it’s not in English originally and the translation is bad) and is perhaps the single most important work of the whole “killing game” sub-genre.
Elantris by Brandon Sanderson. More about the author than an individual book, but it was the first entry in what is now a very large universe called the Cosmere. It can arguably be pointed to as the beginning of a major change to the landscape of the fantasy genre.
Aside from the massive empire that Brandon has built over the years from his books, he also brought the concept of a hard magic system to the forefront of the fantasy genre. He even established what are called “Sanderson’s Laws” on the subject. For example, Sanderson’s First Law states “an author’s ability to solve conflict with magic is directly proportional to how well the reader understands said magic”
I believe there had been authors who had done magic systems that had specific rules and limitations before, but none achieved his level of success, nor had any really taken such an academic approach to writing and attempted to define specific concepts and writing elements. Now almost any fantasy fan can tell you what the difference is between a hard and soft magic system.
Multitudes of authors followed in his footsteps and began defining their magic systems in more detail. Sanderson has achieved international fame and built a multi-million dollar empire. Most recently, he held the single largest kickstarter campaign in history.
So yeah, I’d point to Elantris as the beginning of this.
Argueably much of that was done earlier by robert jordan’s wheel of time which sanderson finished, but that was early 90s affecting.
The Wheel of Time made a massive impact on the fantasy genere. Arguably the biggest shift since LOTR. Sanderson has spoken about loving those books and rereading them endlessly as a kid. It might be too far to say Sanderson is a Jordan product as they never met. But the influence is hard to overstate.