Someone told me this weekendthat a famous fantasy author avoided reading fantasy while he wrote, as he was scared he'd accidentally plagiarise...
My response was that my memory is so cluttered with 45 years of reading, I am sure that everything I write is somehow derivative. It's usually just very well mixed. And that fellow Pratchett plagiarised my unpublished ideas... of course he didn't, but I have seen both myself and other authors evolve similar ideas , independently. I've written a book which would have me accused of plagiarising TP... Only I wrote mine 5 years before he did. And no he did NOT steal my idea. But we both read - obviously - many of the same books. Everything is derivative, in a way.
They went on to say that well, perhaps the answer was just not to read any fantasy. I can happily say that this is a very bad idea. There are a number of authors in literary fiction who've proved this very well. People like Winterson and Atwood who put a lot of effort into saying publically how rubbish that squids in space 'sci-fi' stuff is (which, um, suggests they're terribly well read in the genre, doesn't it?), and then proceed to write sf (which just because it has robots and space ships doesn't make it sci-fi, as they put it) very, very badly. It's not their writing skills per se. It's their ignorance of the conventions of the genre. Now, genre conventions -- and literary fiction has its own set -- are evolved traditions, and like most evolved traditions developed from a seed of common sense wisdom at the time. All genre conventions - whether in murder mystery, literary fiction, biographies or fantasy - drag along a lot of garbage which meant something back when they evolved. And they also carry the elements which make the genre work for audiences. Yep, there may be better ways of connecting with the audience. But it is statistically improbable that you're going to come up with a better way from scratch. You're far more likely to get there by disposing of elements of the present convention which seem outdated, and retaining that which works. And yes, of course you should read other genres, and borrow from them. But an entire transplant of murder mystery (which tends to be plot heavy) into epic fantasy (which tends to be character heavy) is not going to work for the readers who love that genre. So thinking you're going to stun habitual fantasy readers with your new fantasy novel if you've never read any -- is wishful thinking most of the time. It could work. But it probably won't. If you're that lucky, buy a lotto ticket instead of writing. It's quicker and just as likely. Otherwise: read a lot of what you want to write.
That said: a lot of sf and fantasy is very closely derivative. And a lot of it is derivative of derivative. 'Oh no, not another collect the tokens Tolkein-clone... ' So: does the field get tired and need something new? Yep, of course. But here is the kicker -- 'new' is actually a bitch to sell to editors and even to readers. It has to really shine very, very bright. Otherwise... well the field actually loves 'old' - especially if you can put a new twist to it and give it a good voice.