Stunner - Improving called element field for Reusable sub-process

The Missing the following capabilities in the called element field for the Reusable Sub-process:

• Use of (magic) strings
  Eg:

  #{....}
  

• Hability to unset the field value
  • although it's clear that process will not execute until the value is being set
  • warning messages during validation/save will still appear too

UX: See UX requirement from the field here