Did anything like this get implemented?
I think the easiest and most powerful thing would be to add a "workflow" folder option to the context menu within a workflow (in addition to triggers, inputs, etc.), that lets you choose from all of the "built in" workflows. Then you can integrate any existing built-in workflow into our own workflow. When built-in workflows have default actions (e.g., <return> in a bookmark search automatically opens the URL in a browser), just put that as an uneditable action coming out of the workflow. That way you can add other options easily without interfering with existing behavior. It would require no other UI changes to all of the built-in workflows.