Anybody who's been in a bookstore in the last ten years knows that people know what they want, and authors are more than happy to give it to them.
When Harry Potter came out, we were smashed with 5,000 other titles highlighting the in's and out's of the wizarding world told from various perspectives, some more successful than others, none of which touching JK Rowling's runaway hit.
She's the only one that could spin movie deals and merchandising unseen since KISS made everything from lunchboxes to condoms.
Even now, we see spin-off type genres I'd consider fitting like, and Percy Jackson and the Olympians and The Sorcerers Apprentice hitting box-offices at the moment.
Twilight is another beast entirely. Bella Swan and some dark character named Edward have become a generation's touchstones, spawning an entire genre usually referred to as Paranormal Romance. Crazy.
It's gotten so bad that editors on website listings outright say that if they get another vampire romance novel submission, they're apt to drink the magic Kool-Aid and call it quits.
I can't blame them.
It begs the question - which is better?
To pave bravely on in a genre that has been saturated with every Jim, Bob and Sue crossing editors desks with similar queries, or to try something new that isn't known to sell well?
Hell, no one ever would have thought the journey of a nerdy teen in a magical world based on a typical Hero's Journey framework would have the success it did. Likewise, in a world of tanned celebs, nobody could have guessed a pasty Robert Pattinson would become a sex symbol. Both authors crossed into territory previously unknown (at least to the mainstream reader) and hit the jackpot.
It must be taken into account that both Twilight and the Harry Potter series were targeted towards the Young Adult market - impressionable and seeking something new - which was obviously the wise choice.
I guess at the end of the day, we'll just have to sit back and wait to see what new genre will emerge.