I read someone else theorizing that Hoopa would be in the Chamber of Emptiness, and I agree with you and that person in regards to that. When you look at the locations of all the mega stones, it wouldn't make sense if that entire cave was created
just for a mega stone. (Although it could be a callback to
Hoenn's Scorched Slab, but I have higher expectations for it.) I should probably also mention that I'm purposely avoiding spoilers in regards to how these event legendaries are actually obtained... so if this idea is already proved wrong then just dismiss this. XD
Instead of feeling disappointed, I think you should feel excited... speculating--and in your case, experimenting--is part of the fun. :) When the events do come out, it'll be fun to compare how close you guessed right. (Once again, if there's already proof of how the events will play-out, then just ignore this. >.> )