Yes, it is possible to traverse and display the contents of the variables passed to a theme (or localisation). I also agree it is an excellent idea to have a developers custom theme (and localisation) which does this. This would be a valuable resource when starting to develop a custom theme (or localisation) and also during debugging.
For an example if displaying variables see Localisation: GST/VAT worksheet programming guide - #4 by Patch In particular the exploration of the variable “objects”
The display control is easier in localisation but variable access is the same. Liquid is designed to allow access only to the information passed to the user procedure. The content of the data structure passed depends on what data has been entered into the current business, report settings, and which report is currently being displayed.