I'm actually a fan of the Pocahontas sequence.
It might be fun to see them attempt something from The Incredibles on stage, with a bunch of special effects. Wonder how they could make something like Elastigirl work? Gimmicked sleeves that lock into hidden sleds in the background? Have her stay close to the wall and use digital projection? On the other hand, I'm not sure something with superhero/sci-fi elements like The Incredibles would even be a good fit for the more classic looking Fantastmic sequences.
Would be cool to incorporate some limited elements of live musical performance into the show, like Mickey's Magical Map does at Disneyland.
The Maleficent dragon cannot be big enough. Triple it in size, if possible.
Could be neat for them to try to do a Lilo and Stitch surfing thing. Maybe have half a dozen jetski-type vehicles, very low, painted black with its driver wearing all black, and then have a brightly-colored day-glo surfboard on top of it with a brightly-attired actor on top of it, to give the illusion that the "surfer" is the one making the thing move around.
Jetboards would probably be too fast and too difficult for the performers to safely control in the darkness.
Moana would be a good fit too, if we want something on the water.
Not only could they do something with trimarans in the lagoon, but what if they were to do another "miniboss" villain on stage like Kaa and build a Tamatoa, complete with bright, onboard lighting effects. This would be a large, multi-person puppet, I guess. With disco balls.