A new book listing has appeared on Amazon with the title "My Little Pony: A New Adventure". The description for it is copy pasted directly from the synopsis we posted back during Hasbro's press event. It also states that it's a direct copy of the movie in the expanded description.
Considering past releases, this could be the actual movie title. We will have to wait for confirmation, but the previous tie-in books for the g4 movie a few years ago adopted the movie title too.
Thanks to Manamori and Ayu for the heads up!