Treasure Planet

Atlantis has been seen, so I'm assuming Treasure Planet would be on the docket. Just in case I'm wrong and it hasn't been considered, this is my ultimate recommendation!

Treasure Planet and Atlantis are commonly associated with one another because they are considered the last great 2d masterpieces created by Disney, using 3d techniques to enhance the viewing experience before the transition to full 3d animation. As such, they were both under-marketed by Disney in an effort to kill their box office returns. They succeeded in doing so, but the movies thrived amongst those who actually saw them. They gained heavy cult followings and are now considered the last true Disney classics from the early 2000s and the bookend to an era. 

 Treasure Planet, a space adventure reimagining of the story "Treasure Island", is an absolute 'must watch'.