Opeth has been giving us roughly the same style of music since Still Life, though. Sure, Watershed showcases their versatility more than their past albums, but you can definitely tell it's Opeth just by the way the songs are structured and the riffs are written. The only thing that really changed is that Watershed can't be described as "a progressive death metal album with a few gentle moments scattered here and there."
All their old styles have been put on display with equal prominence this time rather than one style overpowering the others. They didn't do anything really "new" on Watershed, though, unless you count that as new.
Porcupine Tree is another band that I always hear people say can't be catergorized, but it's the same thing there. They/Wilson started off as a psychedelic outfit (just like Opeth started off with huge Black metal influences), then they went to awesome pop rock (just like Opeth went on to a more Death metal sound with My Arms, Your Hearse), and then they added some metal to their sound (like Opeth started incorporating more soft moments in Still Life). But Porcupine Tree's last 3 albums have been the same style, and that style is merely a mixing of all of Wilson's past styles. Porcupine Tree hasn't done anything to truly revamp their sound since In Absentia, and even that album was just a blending of Wilson's old stuff with some metal thrown in.
Not dissing either Opeth or Porcupine Tree here. Both of those bands are in my all-time favorites list. But neither are bringing anything new to the music scene anymore. Not that they really need to... just because they aren't revolutionizing the music world like they did in the past doesn't mean the new stuff isn't good. Both Watershed and Fear of a Blank Planet are among my favorites from each band.