[sciurius]

For the upcoming 5.0 version of the ChordPro standard I intend to add the following directives:

• artist
  
• composer
  
• key
  
• time
  
• tempo
  

I see no problem to add 'arranger' as well.
Also

To facilitate using custom extensions for application specific purposes, any directive with a name starting with x_ should be completely ignored by applications that do not handle this directive. In particular, no warning should be generated when an unsupported x_directive is encountered.

It is advised to follow the x_ prefix by a tag that identifies the application (namespace). For example, a directive to control a specific pedal setting for the MobilsSheetsPro program could be named x_mspro_pedal_setting.