Okay, got it. So angels aren't tied to specific bloodlines, it's just that certain bloodlines are more "compatible" and preferable to others. I did remember Nick as Lucifer's vessel in Season Five, but I always assumed he had the right bloodline for it (albeit not one as pure as Sam's).
I agree that having Ezekiel be yet another bad guy angel with an ulterior motive would be boring. Hopefully for once the Winchesters have encountered another angel who's actually on the level.