I saw this on another video, so take this with a grain of salt. But, it is the Rumors board, so alas...
I believe the original vision was for a merry-go-round, perhaps with a 360vision screen, and effects/animations to sort of bring things to life. Which totally makes sense for a Mary Poppins attraction.
Honestly, I thought when it was announced that there wasn't even going to be an attraction. That it was just going to be sort of an immersive re-creation of Cherry Tree Lane for us nostalgia types who just want the feel of being there. Which to be honest? That was the part I was looking forward to anyway. Admiral Boom? One-Man-Band outside? Chalkprints on the pavement? I would have been positively giddy.