I think it's a pretty fair statement to say that the modern fantasy genre is pretty much a creation of J.R.R. Tolkien and his series of fantasy novels. Yes, there might be other influences, depending on the franchise, but all of the common tropes were started by Tolkien. Orcs and goblins, probably led by a Dark Lord? Check. Burly dwarves and magical elves, long-time rivals with each other but both ultimately "good guys"? Check? Humanity living in a faux-medieval Western European sort of culture, locked in technological stasis for thousands of years? Check? Magic being an important part of the world? Check, check, check. Again, there may be some variations depending on the franchise (World of Warcraft and The Chronicles of Narnia have more races, A Song of Ice and Fire has fewer races, there might be more moral ambiguity in the story if it's more recent, etc), but most modern fantasy franchises are at least vaguely cosmetically similar to Middle Earth.
However, concurrent with Tolkien's first writings, an American author was building his own fantasy world. Robert Howard, a writer for various pulp magazines, had created Conan the Barbarian and his Hyborian Age. He was a writer of "low fantasy" (a retroactive term, I believe, to compare it to Tolkien's own "high fantasy"). While elements of Howard's works inspired others in the future (Gary Gygax, co-creator of Dungeons and Dragons, said he preferred Howard's sword-and-sorcery to The Lord of the Rings), his fantasy world was very cosmetically different than Tolkien's work. His was a world that featured dying civilizations, ancient monsters and barbarians. However, while Conan the Barbarian remains popular, most modern fantasy authors have opted to emulate Tolkien rather than Howard.
But what if Howard and Tolkien's situations had been reversed? What if Tolkien had died in an accident during World War II? The Hobbit is still published and successful (though it's first editions had some important differences to later editions), and someone (C.S. Lewis? Christopher Tolkien?) might at some point make his notes public, detailing a vaguely flesh-out fantasy world with a connection at best tenuous to The Hobbit, but that's all we get from him. Meanwhile, Robert Howard is able to overcome his depression and doesn't commit suicide in 1936. At the time it seemed like he was finally about to make his big break in the world of novels, moving up from the pulps. Say he writes some highly successful (non-fantasy) novels in the late 1930's and early 1940's, enough to live a comfortable life. By the late 1940's in this scenario, his attention turns back to Conan and the Hyborian Age. Though perhaps not as deeply knowledgeable as Tolkien, Howard had a similar fascination with historical civilizations and languages, and sets out to write an epic series of novels set in his fantasy world. Published sometime in the 1950's, they become to the old Conan pulp stories what The Lord of the Rings was to The Hobbit, and are equally as influentially in at least American popular culture.
So my question is, how do these changes effect the fantasy genre, in book, film, and various sorts of games over the years? What would the genre look like if Howard "low fantasy" not Tolkien's "high fantasy", had been dominant?
(BONUS: What if H.P. Lovecraft also lives in this scenario? Say a now successful Howard is able to help out his friend's financial situation, Lovecraft continues his writings, and eventually breaks out into novels by the 1950's, with this signature "weird fiction" stories? Lovecraft has had some influence in modern speculative fiction - strong in sci-fi and horror, peripheral in fantasy - so what if the effect is multiplied?)