Dynamically loading terms in a term reference tree from a huge taxonomy so users may select nodes as options
You can try some contrib modules:
Hierarchical Select  


  Module defines the "hierarchical_select" form element, which is a
  greatly enhanced way for letting the user select items in a hierarchy. Demo


Simple hierarchical select  


  Defines a new form widget for taxonomy fields to select a term by
  "browsing" through the vocabularies hierarchy.


You can use it with High-performance JavaScript callback handler 

Also you can try https://www.drupal.org/project/views_jstree