Rock n Roll or Metal would not exist if not for experimentation. We would all be listening to jazz. Every Priest album has  personality, beuase they are all different.
Nostradomus is great, I love it. There is no comparison to it. Nostradomus is not my favorite JP album, but it is the one that captures me.

By the way there is not any generic form of heavy metal. Thrash would not exist if it was.

AC/DC is a great band, they are one of my favorite bands. But every one of their albums sound the same. I listen to them in small portions. I can listen to Priest album after Priest album for months. Becuase they are different. 

I also thought metal was about not being the same. Standing out from the crowd. Why be generic? If you think Priest are going to go generic than "You've got another thing comin' ."