To properly extend ARexx, you must understand ARexx. ARexx is like very few other popular languages. It has been called "another basic" and "A CMS scripting language". Neither of these is correct, though they do describe aspects of ARexx.
As an extension to ARexx, an applications interface must fit into ARexx. Before that can be done properly, the designer must understand what functions ARexx is used for, and how it is used for them. Once the function is understood, the form will be easier to design.
The poor interfaces in current systems indicate that others have not considered this. See some evaluations of current interfaces for details.
Rexx has aspects of a:
On to goals of a design, back to goals of this document, or up to the contents.
Mike W. Meyer