The tough part is that Disney no longer does simple stuff for the most part. I get the impression that everything has to be beyond the basic. The most basic recent attraction they've done is the Alien Swirling Saucers. It works because of the theme in the area.
Mary Poppins is too life like to be combined with a very ordinary ride mechanism. However, adding more capacity is a good thing for the area.
When I think of Mary Poppins, I think of the umbrella and floating to the ground. Certainly a spinner with hydraulic lifting umbrellas could work well, especially if they pair it with Dumbo style queue. Live stage music. Mix in a food and beverage option and you have a recipe to eat up crowds and take in some money.
It would be a mass of strollers, with mom and dad grooving to the music, sipping a beverage, rockin the stroller while youngest sleeps and the next oldest is spinning around in circles.