@Ubersoft: Apologies for mis-citing you!
"In fact, I'm pretty sure you could hold the collected works of the ancient world on a few shelves. Compare that to the vast libraries we have now."
I can see how if you separate stories into eras it could look that way. But here's another perspective:
What if instead of drawing a line between the ancient world and the modern one and comparing, we look at it as a continuum? To me our vast libraries don't look original. They look like a multitude of facets of the same very old stories. Whenever our hyoid bones started to allow complex symbolic communication through speech the oral tradition began. We lived in very low population densities where it took time and effort to find the next group of humans. So the stories all looked very similar, and as culture groups grew and dispersed and diversified because of their environments, the stories changed. Then those diversified culture groups met each other again, found different stories, got inspired, and imitated them or altered their own stories.
Follow that continuum to the present day, where with our population explosion and rampant globalization you don't have to walk three days to meet the next group of people. You just hop on a computer and you can find stories from three thousand miles away. So we have this critical mass of ideas and experiences bouncing and ricocheting and shattering into new facets based on our individual, community, and cultural experience. And that is where I see the potential for originality, if you want to call it that. Historical cultures and modern individuals will even come up with the same idea independently. So who gets dibs? Who's not original? Or are we coming up with the same ideas because it's all ultimately coming from the same source? Not questions I have answers to, but ones I ponder.
I'm going to use Winter Rain (which I enjoyed quite a bit of) and Guts and Sass as examples. Your basic premise in Winter Rain is werewolves (I link for anyone reading who isn't already familiar with the story). Some people trace that back to Romulus and Remus, but I think you can go all the way back to the mythologies of hunter-gatherer peoples whose stories teach that they learned how to hunt from wolves. Hunter-gatherer stories are full of shapechangers. Look at Coyote, or Raven. To me any use of the werewolf idea derivative. It's just not as direct as someone writing fanfiction of Winter Rain. I don't think derivative is bad. It just is. Werewolf stories are a dime a dozen. What's different, "original" you could say, about Winter Rain is the depth of culture you show us, and the subtlety of how you reveal it, showing and not telling.
Same thing with Guts and Sass. It's got shapeshifters. Giant, megafauna cat shapeshifters. Hardly original. It's got someone from our world zapped to a magical land and going through culture shock. Hardly original. But these ideas speak to a lot of people, otherwise they wouldn't keep getting used. And my goal is to take these ideas that speak to me, and execute them in a way that actually resonates with my experience of the world.
And as for fanfiction- I've read some fanfiction where I wish they had written the original story, because man, they did it better. And sometimes I think the most original and profound execution is found in the characters and worlds we're already familiar with (archetypes). They're already a part of our cultural consciousness, something we can immediately relate to other people about. You don't have to muck around with world building and all the details because they're already there. And sometimes that means you can really go deeper into the actual story and what significance it has to human experience. Not always. There's some really vapid fanfiction out there. But there's proportionally just as much vapid "original" stuff out there too.