I'd like to stay away from celestial/demonic ideas, personally. We're already having two very important planes, adding in 2-4 more I think would just complicate it. The idea of the plane being odd because of the elder dragon sleeping in it, though, that idea I like.

Different note, I think that manifest is perfect for dreams (and frankly I think wotc could have done it far better than they did, flavor wise). You could also add in sorta removal by adding in the ability to flip back over the opponents unmorphed cards, maybe, banishing things back to the dream.