We have implemented a new field called Connected Dropdown, which has possibility to add an unlimited number of nodes and sub-nodes, in a hierarchical tree.
An example would be Location:
Main nodes: Rogaland, Hordaland
Rogaland: Stavanger, Sandnes, Haugesund
Hordaland: Bergen, Voss
Stavanger: Stokka, Storhaug, Tasta
To expand sub-nodes, hover the mouse over a node. To select, simply click.