Twilight is a phenomenon, no matter what anyone thinks of the books. The chances of this sort of phenomenon repeating itself in the near future is very, very slim.
Paranormal YA's have existed for a long, long time. I remember reading RL Stine, Christopher Pike, and LJ Smith as a teen, and craving those types of stories. They led me to Stephen King, Anne Rice, and more adult fare. Twilight's affect was twofold: 1) It brought the genre to the forefront of the media and to folks who'd never bothered to read it before, and 2) It brought in new readers.
I can personally attest to #2, because I have several friends who, after reading Twilight, have looked for other, similar books. So they discovered paranormal YA because of Twilight. But Twilight did not create or spawn the genre, any more than Laurel K Hamilton spawned the current sexy-vamp trend.
Not only are more new books being published in this genre, but old classics are being reissued. Look at the LJ Smith books--both The Vampire Diaries and Night World series have been reissued in omnibus editions, not to mention TVD will be a CW series this fall. So if Twilight did anything, it reinvigorated an existing genre.
And from what I understand of HP, I can't imagine who it would have made other folks more receptive to Twilight. They seem to be very, very different animals.
Books that cross genres have the potential to do better since they can attract a larger audience. It's all speculation but I'd imagine non-traditionally fantasy agents are now more comfortable selling UF to pubs because the market has increased and the editors they know are more receptive to UF.
