I don't think CO will be a stage in the game because I don't think the game will really advance beyond Sora and Riku coming out of the Dream Worlds and becoming Keyblade Masters. I think that is the logical ending for KH3D, and KH3 will be about Sora as a Master going on the actual quest to stop Xehanort once and for all, while saving all his friends along the way.
While Sora and Riku are undergoing the Mark of Mastery, many events will be occurring in the real worlds, which will set the stage for KH3, but it doesn't mean that Sora and Riku will ever interact with these elements, nor will they have a major influence on the Dream Worlds.
Yes, after watching that particular scene again, I think the CG is hinting at a climatic scene in KH3D. Maybe at the end of the trials a ghostly memory of Master Xehanort prevents then from leaving the Dream World, and Mickey goes in to help them.