Define Subroutine (or Composite Activity) in designer

While implementing more complex workflows in designer I came upon a need to have "reusable groups of activities with standardized inputs and outputs", something that is present in every programming language for obvious needs.

Invoking other workflow is not good enough (at least not for the my target group of users) because
- it lacks easy way to pick which workflow is invoked (have to use id or javascript expression for finding id from name, .....)
- it lacks easy way to fill input "parameters" (data that is passed into it)
- it lacks easy way to find out output "parameters" (data that is passed out of it)

One way to achieve my needs (with some polishing) would be to enhance way workflow can be invoked from another workflow.

Other way that comes into my mind is implementing capabilities to design Composite Activity in designer (AFAIK they must be defined in code at the moment).
This would have to go hand in hand with capability to enable calling Composite Activity "defined" in one workflow from another workflow.

Is there a way I missed to achieve something like this with current Elsa?
If not, is there a plan to add this in some future Elsa version?