Some observations about epic fantasy tropes after reading too much of it:
The chosen one trope won't die because readers don't actually want it to. They say they're tired of it. Then they stay up until 3am reading about a farmboy with a destiny. The trope works because it answers a question most people carry around and never say out loud: what if I mattered more than my circumstances suggest.
Tolkien said "I wisely started with a map." Most people quote this as worldbuilding advice. I think it's something else. He was a philologist who invented languages for fun, and the stories grew out of those languages, and the map grew out of the stories. He didn't plan Middle-earth. He discovered it. The trope of the detailed fantasy map exists because of one man's obsession with linguistics, which is a strange and wonderful origin for a genre convention.
The dark lord as a faceless evil worked for a long time. It stopped working around the time readers started asking "but why is he evil." Joe Abercrombie understood this better than anyone. In The First Law, Bayaz is a wizard who looks like a mentor and acts like a hero and turns out to be the most dangerous person in the story. The trope of the wise wizard gets used against the reader.
Epic fantasy conventions around prophecy tend to fall apart when the prophecy is too specific. "The one born under a red moon will slay the shadow king" gives you nowhere to go. The prophecies that work are the ones vague enough to be wrong.
There's a reason the fellowship trope, the band of mismatched companions on a quest, keeps showing up in every generation of epic fantasy. It mirrors how people actually form bonds under pressure. The dwarf and the elf don't become friends because they're compatible. They become friends because they nearly died together on a mountain and that kind of thing rearranges your priorities.
The worst epic fantasy trope is the training montage where the protagonist becomes skilled without cost. The best version is Patrick Rothfuss in The Name of the Wind, where Kvothe spends years learning sympathy at the University and every lesson costs him money he doesn't have, sleep he can't afford, and relationships he keeps damaging because he's too proud to ask for help. The learning is tangled up with the living. You can't separate them.
I'm still not sure whether epic fantasy needs to be long. The convention says yes, the longer the better, and readers do seem to want that. But I wonder sometimes if the length is the point or just a side effect of writers who fell in love with their own worlds and couldn't stop building rooms in a house that was already big enough to live in.
Tad Williams wrote Memory, Sorrow, and Thorn with a scullion as the protagonist before "ordinary person thrust into extraordinary circumstances" became the default epic fantasy setup. George R.R. Martin has said publicly that Williams' work inspired him to write A Song of Ice and Fire. The trope that launched the most famous fantasy series of the last thirty years came from a book most people haven't read.
Magic systems in epic fantasy fall into two camps and neither is better than the other. There's the Rothfuss approach where sympathy follows the laws of thermodynamics and every spell has a measurable cost. And there's the Tolkien approach where Gandalf does something luminous and unexplained and you just accept it because the story has earned your trust. The trope of "hard magic vs. soft magic" is really a question about how much mystery you want your reader sitting with.