To me I think any new additions should be in line with the European storybook theme that permeates Fantasyland. Additions from Tangled, Beauty and the Beast, The Hunchback of Notre Dame, and even Brave would fit really nicely.
On the other hand, I've always felt that the Agrabah palace is out of place with the rest of the miniatures and sort of sticks out like a sore thumb; I think the same would be true of additions based on Mulan, The Lion King, or any of the Pixar movies that have a more contemporary setting (i.e. most of them).
I think Rapunzel's tower would be a really nice addition, and I actually quite like the idea of replacing the Cave of Wonders and expanding the attraction underneath the old skyway station.