I actually really liked the ending a lot. Since I took the whole thing as a metaphor for the process of making a horror film in the creative slump that has been American horror over the past few decades, with obvious nods to the Japanese for picking up the slack there. So, with that in mind, the "ritual" serves as the blueprint for lazy horror film making. I would suspect that the Japanese have a different ritual, considering that their ritual takes place in a pre-teen girls' school. And the rising of "the ancient ones" is just the common masses being upset that the film didn't follow the standard tropes. So, if you want to see what the ancient ones looked like, just look at the audience.
In fact, the only thing that annoyed me was the inclusion of the cenobite character, since Hellraiser, even as a series, has tried to hard to step away from those tropes.
So, in many ways, the "horror" aspect of the film really only comes from the uncomfortable insinuation that we as a people need to see these characters violently punished out of some form of primal bloodlust. In many ways, this film reminds me more and more of films like Peeping Tom in that respect.
Yeah, it's a bit precious in that statement and I would accept the criticism that Whedon hasn't really earned the right to make a statement like that, as opposed to someone like Carpenter or Cronenberg whose careers were affected by their attempts to expand beyond standard tropes. But, I do still think that the statement is valid when you look at the success of the structure in American horror.